As filed with the Securities and Exchange Commission on February 9, 2000.
                              Registration No. 333-
         -------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                                 ---------------

                                    FORM S-8
                             Registration Statement
                                      Under
                           The Securities Act of 1933
                                 ---------------

                         THERMO INSTRUMENT SYSTEMS INC.
             (Exact name of registrant as specified in its charter)
                                 ---------------

      DELAWARE                                                   04-2925809
(State or other jurisdiction of                             (I.R.S. Employer
 incorporation or organization)                        Identification Number)

                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046
               (Address of Principal Executive Offices) (Zip Code)


                 THERMO VISION CORPORATION EQUITY INCENTIVE PLAN

   THERMO ELECTRON CORPORATION - THERMO VISION CORPORATION NONQUALIFIED STOCK
                                   OPTION PLAN

  THERMO INSTRUMENT SYSTEMS INC. - THERMO VISION CORPORATION NONQUALIFIED STOCK
                                   OPTION PLAN

 THERMO ELECTRON CORPORATION DIRECTORS STOCK OPTION PLAN (THERMO VISION SHARES)

    THERMO INSTRUMENT SYSTEMS INC. DIRECTORS STOCK OPTION PLAN (THERMO VISION
                                     SHARES)

                           (Full Titles of the Plans)

                          Sandra L. Lambert, Secretary
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046
                     (Name and Address of Agent for Service)

                                  (781) 622-1000
          (Telephone Number, Including Area Code, of Agent For Service)
<PAGE>

                                   Copies to:
                    Seth H. Hoogasian, Esq., General Counsel
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046

                                 ---------------

                         CALCULATION OF REGISTRATION FEE


 Title of                                    Proposed Maximum
securities         Amount        Proposed         Aggregate           Amount of
  to be             To be        Maximum     Offering Price        Registration
registered       registered     Offering                                 Fee
                                 Price Per
                                   Share
Common Stock,
 $.10 par          324,416       $16.53125     $5,363,002 (2)         $1,416
value per           shares          (2)
 share              (1)


      In  addition,  pursuant to Rule 416(c) under the  Securities  Act of 1933,
this Registration Statement also covers an indeterminate number of shares of the
Registrant's  Common  Stock as may be issuable in  connection  with  adjustments
under the employee  benefit plans described herein to reflect certain changes in
the Registrant's capital structure, including stock dividends or stock splits.

(1)   The shares  registered  hereunder  are divided among the various plans set
      forth below :

Thermo Vision Corporation Equity Incentive Plan: 239,886 shares

Thermo Electron Corporation - Thermo Vision Corporation Nonqualified Stock
Option Plan: 40,694 shares

Thermo Instrument Systems Inc. - Thermo Vision Corporation Nonqualified Stock
Option Plan: 35,833 shares

Thermo Instrument Systems Inc. Directors Stock Option Plan (Thermo Vision
Shares): 2,000 shares

Thermo Electron Corporation Directors Stock Option Plan (Thermo Vision Shares):
6,003 shares
<PAGE>


(2)   Estimated  solely  for  the  purpose  of  calculating  the  amount  of the
      registration  fee in accordance  with Rule 457(h) under the Securities Act
      of 1933. The calculation of the proposed maximum aggregate  offering price
      has been based upon (1) the  registration  hereunder  of an  aggregate  of
      324,416  shares  and (2) the  average  of the high and low  sales  prices,
      $17.00 and $16.0625, respectively, of the Registrant's Common Stock on the
      American   Stock   Exchange  on  February  4,  2000  as  reported  in  the
      consolidated transaction reporting system.



<PAGE>



                                     PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The information  required by Part I is included in documents sent or given
to the respective participants in the plans pursuant to Rule 428(b)(1) under the
Securities  Act of 1933,  as amended  (the  "Securities  Act").  As used in this
Registration  Statement,  the terms  "Registrant"  or "Company"  refer to Thermo
Instrument Systems Inc.



                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

      The Registrant is subject to the informational and reporting  requirements
of Sections 13(a),  13(c), 14 and 15(d) of the Securities  Exchange Act of 1934,
as amended (the  "Exchange  Act"),  and in accordance  therewith  files reports,
proxy  statements  and  other  information  with  the  Securities  and  Exchange
Commission (the "Commission").  The following documents,  which are on file with
the Commission, are incorporated in this Registration Statement by reference:

            (a)   The  Company's  Annual Report on Form 10-K for the fiscal year
                  ended January 2, 1999, as amended.

            (b)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on January 8, 1999.

            (c)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on March 9, 1999.

            (d)   The  Company's  Quarterly  Report on Form 10-Q for the quarter
                  ended April 3, 1999.

            (e)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on May 25, 1999.

            (f)   The  Company's  Quarterly  Report on Form 10-Q for the quarter
                  ended July 3, 1999.

            (g)   The  Company's  Quarterly  Report on Form 10-Q for the quarter
                  ended October 2, 1999.

            (h)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on February 1, 2000.

            (i)   The  description of the Common Stock which is contained in the
                  Company's  Registration  Statement on Form 8-A filed under the
                  Exchange Act, as such  description may be amended from time to
                  time.
<PAGE>



      All reports or proxy  statements filed by the Company pursuant to Sections
13(a),  13(c),  14 and 15(d) of the Exchange Act  subsequent to the date of this
Registration  Statement  and prior to the filing of a  post-effective  amendment
that  indicates  that all  securities  offered  herein  have been sold,  or that
deregisters  all such securities  then remaining  unsold,  shall be deemed to be
incorporated by reference in this Registration Statement and to be a part hereof
from the respective dates of filing of such documents.

Item 4.  Description of Securities.

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.

     The  validity of the Common  Stock  offered  hereby has been passed upon by
Seth H.  Hoogasian,  Esq.,  General Counsel of the Company.  Mr.  Hoogasian is a
full-time  employee of Thermo  Electron  Corporation  ("Thermo  Electron"),  the
majority  stockholder  of the  Company,  is an officer of the Company and Thermo
Electron,  and owns or has the right to acquire  22,331  shares of Common Stock,
379,785  shares of the  common  stock,  $1.00 par  value  per  share,  of Thermo
Electron, and 20,833 shares of the common stock of the Company's subsidiaries.

Item 6.  Indemnification of Directors and Officers.

      The Delaware  General  Corporation  Law and the Company's  Certificate  of
Incorporation  and By-Laws  limit the  monetary  liability  of  directors to the
Company and to its stockholders and provide for indemnification of the Company's
officers and directors for  liabilities and expenses that they may incur in such
capacities.  In general,  officers and directors are indemnified with respect to
actions  taken in good faith in a manner  reasonably  believed  to be in, or not
opposed to, the best  interests of the Company and, with respect to any criminal
action or proceeding,  actions that the  indemnitee  had no reasonable  cause to
believe were unlawful. The Company also has indemnification  agreements with its
directors and officers that provide for the maximum  indemnification  allowed by
law.

      The Company has an  insurance  policy  which  insures  the  directors  and
officers of the Company and its subsidiaries  against certain  liabilities which
might be incurred in connection with the performance of their duties.

Item 7.  Exemption from Registration Claimed.

      Not Applicable.
<PAGE>

Item 8.  Exhibits.

      The Exhibit Index  immediately  preceding the exhibits is attached  hereto
and incorporated herein by reference.

Item 9.  Undertakings.

      (a)   The undersigned Registrant hereby undertakes:

            (1) To file,  during any  period in which  offers or sales are being
made, a post-effective amendment to this registration statement:

                  (i)    To include any prospectus required by Section 10(a)(3)
                           of the Securities Act of 1933;

                  (ii) To reflect in the  prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent  a  fundamental  change  in the  information  set  forth  in the
      registration  statement.  Notwithstanding  the foregoing,  any increase or
      decrease in volume of  securities  offered (if the total  dollar  value of
      securities  offered  would not exceed that which was  registered)  and any
      deviation from the low or high end of the estimated maximum offering range
      may be  reflected  in the form of  prospectus  filed  with the  Commission
      pursuant  to Rule 424(b) if, in the  aggregate,  the changes in volume and
      price  represent no more than 20 percent  change in the maximum  aggregate
      offering price set forth in the "Calculation of Registration Fee" table in
      the effective registration statement;

                  (iii) To include any material  information with respect to the
      plan  of  distribution  not  previously   disclosed  in  the  registration
      statement or any material change to such  information in the  registration
      statement.

      Provided,  however,  that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply
if the  information  required to be included in a  post-effective  amendment  by
those paragraphs is contained in periodic reports filed with or furnished to the
Commission  by the  Registrant  pursuant  to Section 13 or Section  15(d) of the
Securities  Exchange  Act of 1934  that are  incorporated  by  reference  in the
registration statement.

            (2) That,  for the purpose of  determining  any liability  under the
Securities Act of 1933, each such post-effective amendment shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such  securities at that time shall be deemed to be the initial bona
fide offering thereof.

            (3)  To  remove  from  registration  by  means  of a  post-effective
amendment  any of the  securities  being  registered  which remain unsold at the
termination of the offering.
<PAGE>

      (b) The undersigned  Registrant  hereby  undertakes  that, for purposes of
determining  any liability  under the Securities Act of 1933, each filing of the
Registrant's  annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable,  each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the  registration  statement shall be
deemed to be a new  registration  statement  relating to the securities  offered
therein,  and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.


      (c)  Insofar  as  indemnification   for  liabilities   arising  under  the
Securities Act of 1933 may be permitted to directors,  officers and  controlling
persons of the Registrant  pursuant to the foregoing  provisions,  or otherwise,
the  Registrant  has been  advised  that in the  opinion of the  Securities  and
Exchange  Commission such  indemnification is against public policy as expressed
in the Act and is,  therefore,  unenforceable.  In the  event  that a claim  for
indemnification  against  such  liabilities  (other  than  the  payment  by  the
Registrant of expenses  incurred or paid by a director,  officer or  controlling
person of the  Registrant  in the  successful  defense  of any  action,  suit or
proceeding)  is  asserted by such  director,  officer or  controlling  person in
connection with the securities being registered,  the Registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit  to a  court  of  appropriate  jurisdiction  the  question  whether  such
indemnification  by it is against public policy as expressed in the Act and will
be governed by the final adjudication of such issue.


<PAGE>


                                   SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies  that it has  reasonable  grounds to believe  that it meets all of the
requirements  for  filing  on Form S-8 and has  duly  caused  this  Registration
Statement  to be  signed  on its  behalf  by  the  undersigned,  thereunto  duly
authorized, in the City of Waltham,  Commonwealth of Massachusetts,  on this 9th
day of February, 2000.

                                   THERMO INSTRUMENT SYSTEMS INC.


                                   By:  /s/ Earl R. Lewis
                                        ----------------------------------
                                        Earl R. Lewis
                                        President and Chief Executive
                                        Officer


                                POWER OF ATTORNEY

      Each of the  undersigned  Directors  and  Officers  of  Thermo  Instrument
Systems Inc. hereby appoints Theo  Melas-Kyriazi,  Paul F. Kelleher,  Kenneth J.
Apicerno,  Seth H.  Hoogasian and Sandra L. Lambert,  and each of them, his true
and lawful  attorneys-in-fact  and agents, with full power of substitution,  for
him and in his name, place and stead, in any and all capacities, to sign any and
all  amendments  (including  post-effective  amendments)  to  this  Registration
Statement, and to file the same, with all exhibits thereto, and all documents in
connection therewith, with the Securities and Exchange Commission, granting unto
said attorneys-in-fact and agents, and each of them, full power and authority to
do and perform each and every act and thing  requisite  and necessary to be done
in and about the  premises,  as fully to all intents and purposes as he might or
could  do  in  person,   hereby   ratifying   and   confirming   all  that  said
attorneys-in-fact  and  agents  or any of them,  or their or his  substitute  or
substitutes, may lawfully do or cause to be done by virtue thereof.

      Pursuant  to  the  requirements  of  the  Securities  Act  of  1933,  this
Registration  Statement  has been signed below by the  following  persons in the
capacities and on the dates indicated:

 Signature                           Title                       Date


/s/ Earl R. Lewis             President, Chief Executive    February 9, 2000
-------------------------     Officer and Director
Earl R. Lewis

<PAGE>


/s/ Theo Melas-Kyriazi        Chief Financial Officer       February 9, 2000
-------------------------
Theo Melas-Kyriazi

/s/ Paul F. Kelleher          Chief Accounting Officer      February 9, 2000
-------------------------
Paul F. Kelleher

/s/ Arvin H. Smith            Chairman of the Board         February 9, 2000
-------------------------
Arvin H. Smith

/s/ George N. Hatsopoulos     Director                      February 9, 2000
-------------------------
George N. Hatsopoulos

/s/ John N. Hatsopoulos       Director                      February 9, 2000
-------------------------
John N. Hatsopoulos

/s/ Richard F. Syron          Director                      February 9, 2000
-------------------------
Richard F. Syron

/s/ Polyvios C. Vintiadis     Director                      February 9, 2000
-------------------------
Polyvios C. Vintiadis





<PAGE>


                                  EXHIBIT INDEX


Exhibit
Number            Description


5                 Opinion of Seth H. Hoogasian, Esq.

23.1              Consent of Arthur Andersen LLP

23.2              Consent of Seth H. Hoogasian, Esq.
                  (contained in his opinion filed as
                  Exhibit 5).

24                Power of Attorney (see  signature  pages to this  Registration
                  Statement).



<PAGE>





                                                                      EXHIBIT 5



                         Thermo Instrument Systems Inc.
                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046




                                                      February 9, 2000

Thermo Instrument Systems Inc.
81 Wyman Street
Waltham, Massachusetts 02454-9046

Re:   Registration Statement on Form S-8
      Relating to 324,416 Shares of the Common Stock,
      $.10 par value, of Thermo Instrument Systems Inc.

Dear Sirs:

      I am  General  Counsel  to Thermo  Instrument  Systems  Inc.,  a  Delaware
corporation  (the  "Company"),  and have acted as counsel in connection with the
registration  under the  Securities  Act of 1933,  as amended,  on Form S-8 (the
"Registration Statement"), of 324,416 shares of the Company's common stock, $.10
par value per share (the  "Shares")  subject to the  Thermo  Vision  Corporation
Equity  Incentive  Plan,  the  Thermo  Electron   Corporation  -  Thermo  Vision
Corporation Nonqualified Stock Option Plan, the Thermo Instrument Systems Inc. -
Thermo Vision  Corporation  Nonqualified  Stock Option Plan, the Thermo Electron
Corporation  Directors Stock Option Plan (Thermo Vision Shares),  and the Thermo
Instrument  Systems Inc.  Directors  Stock Option Plan  (Thermo  Vision  Shares)
(collectively, the "Plans").

      I or a member of my legal staff have  reviewed the  corporate  proceedings
taken by the Company  with respect to the  authorization  of the issuance of the
Shares.  I or a member of my legal  staff have also  examined  and  relied  upon
originals or copies, certified or otherwise authenticated to my satisfaction, of
all corporate records, documents, agreements or other instruments of the Company
and have made all  investigations  of law and have  discussed with the Company's
representatives   all  questions  of  fact  that  I  have  deemed  necessary  or
appropriate.
<PAGE>

      Based upon and subject to the foregoing, I am of the opinion that:

      1. The Company is a  corporation  validly  existing and in corporate  good
standing under the laws of the State of Delaware.

      2. The issuance and sale of the Shares as contemplated in the Registration
Statement have been duly authorized by the Company.

      3. The Shares,  when issued and sold in accordance  with the provisions of
the respective Plans, will be validly issued, fully paid and nonassessable.


      I hereby  consent  to the  filing  of this  opinion  as  Exhibit  5 to the
Registration Statement.

                                        Very truly yours,



                                        /s/ Seth H. Hoogasian
                                        ----------------------------
                                        Seth H. Hoogasian
                                        General Counsel


<PAGE>


                                                                    Exhibit 23.1






            Consent of Independent Public Accountants
            -----------------------------------------


     As independent public  accountants,  we hereby consent to the incorporation
by  reference  in this  registration  statement  on Form S-8 of our report dated
February 16, 1999  (except with respect to the matters  discussed in Note 18, as
to which  the date is March 1,  1999),  included  in Thermo  Instrument  Systems
Inc.'s Annual Report on Form 10-K for the year ended January 2, 1999, and to all
references to our Firm included in this registration statement.


                                             Arthur Andersen LLP



Boston, Massachusetts
February 4, 2000
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 21842}]
{"company-data": {"conformed-name": "THERMO INSTRUMENT SYSTEMS INC", "cik": "0000795986", "assigned-sic": "3829", "irs-number": "042925809", "state-of-incorporation": "DE", "fiscal-year-end": "0102"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-96495", "film-number": "529465"}, "business-address": {"street1": "81 WYMAN ST", "street2": "PO BOX 9046", "city": "WALTHAM", "state": "MA", "zip": "02454-9046", "phone": "7816221000"}, "mail-address": {"street1": "81 WYMAN ST", "street2": "PO BOX 9046", "city": "WALTHAM", "state": "MA", "zip": "02454"}}
As filed with the Securities and Exchange Commission on February 9, 2000.
                              Registration No. 333-
         -------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549
                                 ---------------

                                    FORM S-8
                             Registration Statement
                                      Under
                           The Securities Act of 1933
                                 ---------------

                         THERMO INSTRUMENT SYSTEMS INC.
             (Exact name of registrant as specified in its charter)
                                 ---------------

      DELAWARE                                                   04-2925809
(State or other jurisdiction of                             (I.R.S. Employer
 incorporation or organization)                        Identification Number)

                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046
               (Address of Principal Executive Offices) (Zip Code)

                 THERMO VISION CORPORATION EQUITY INCENTIVE PLAN

   THERMO ELECTRON CORPORATION - THERMO VISION CORPORATION NONQUALIFIED STOCK
                                   OPTION PLAN

  THERMO INSTRUMENT SYSTEMS INC. - THERMO VISION CORPORATION NONQUALIFIED STOCK
                                   OPTION PLAN

 THERMO ELECTRON CORPORATION DIRECTORS STOCK OPTION PLAN (THERMO VISION SHARES)

    THERMO INSTRUMENT SYSTEMS INC. DIRECTORS STOCK OPTION PLAN (THERMO VISION
                                     SHARES)

                           (Full Titles of the Plans)

                          Sandra L. Lambert, Secretary
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046
                     (Name and Address of Agent for Service)

                                  (781) 622-1000
          (Telephone Number, Including Area Code, of Agent For Service)
Copies to:
                    Seth H. Hoogasian, Esq., General Counsel
                         Thermo Instrument Systems Inc.
                         c/o Thermo Electron Corporation
                                 81 Wyman Street
                                 P. O. Box 9046
                        Waltham, Massachusetts 02454-9046

                                 ---------------

                         CALCULATION OF REGISTRATION FEE

 Title of                                    Proposed Maximum
securities         Amount        Proposed         Aggregate           Amount of
  to be             To be        Maximum     Offering Price        Registration
registered       registered     Offering                                 Fee
                                 Price Per
                                   Share
Common Stock,
 $.10 par          324,416       $16.53125     $5,363,002 (2)         $1,416
value per           shares          (2)
 share              (1)

      In  addition,  pursuant to Rule 416(c) under the  Securities  Act of 1933,
this Registration Statement also covers an indeterminate number of shares of the
Registrant's  Common  Stock as may be issuable in  connection  with  adjustments
under the employee  benefit plans described herein to reflect certain changes in
the Registrant's capital structure, including stock dividends or stock splits.

(1)   The shares  registered  hereunder  are divided among the various plans set
      forth below :

Thermo Vision Corporation Equity Incentive Plan: 239,886 shares

Thermo Electron Corporation - Thermo Vision Corporation Nonqualified Stock
Option Plan: 40,694 shares

Thermo Instrument Systems Inc. - Thermo Vision Corporation Nonqualified Stock
Option Plan: 35,833 shares

Thermo Instrument Systems Inc. Directors Stock Option Plan (Thermo Vision
Shares): 2,000 shares

Thermo Electron Corporation Directors Stock Option Plan (Thermo Vision Shares):
6,003 shares
(2)   Estimated  solely  for  the  purpose  of  calculating  the  amount  of the
      registration  fee in accordance  with Rule 457(h) under the Securities Act
      of 1933. The calculation of the proposed maximum aggregate  offering price
      has been based upon (1) the  registration  hereunder  of an  aggregate  of
      324,416  shares  and (2) the  average  of the high and low  sales  prices,
      $17.00 and $16.0625, respectively, of the Registrant's Common Stock on the
      American   Stock   Exchange  on  February  4,  2000  as  reported  in  the
      consolidated transaction reporting system.
PART I

              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

      The information  required by Part I is included in documents sent or given
to the respective participants in the plans pursuant to Rule 428(b)(1) under the
Securities  Act of 1933,  as amended  (the  "Securities  Act").  As used in this
Registration  Statement,  the terms  "Registrant"  or "Company"  refer to Thermo
Instrument Systems Inc.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

      The Registrant is subject to the informational and reporting  requirements
of Sections 13(a),  13(c), 14 and 15(d) of the Securities  Exchange Act of 1934,
as amended (the  "Exchange  Act"),  and in accordance  therewith  files reports,
proxy  statements  and  other  information  with  the  Securities  and  Exchange
Commission (the "Commission").  The following documents,  which are on file with
the Commission, are incorporated in this Registration Statement by reference:

            (a)   The  Company's  Annual Report on Form 10-K for the fiscal year
                  ended January 2, 1999, as amended.

            (b)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on January 8, 1999.

            (c)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on March 9, 1999.

            (d)   The  Company's  Quarterly  Report on Form 10-Q for the quarter
                  ended April 3, 1999.

            (e)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on May 25, 1999.

            (f)   The  Company's  Quarterly  Report on Form 10-Q for the quarter
                  ended July 3, 1999.

            (g)   The  Company's  Quarterly  Report on Form 10-Q for the quarter
                  ended October 2, 1999.

            (h)   The  Company's  Current  Report  on Form  8-K  filed  with the
                  Commission on February 1, 2000.

            (i)   The  description of the Common Stock which is contained in the
                  Company's  Registration  Statement on Form 8-A filed under the
                  Exchange Act, as such  description may be amended from time to
                  time.
All reports or proxy  statements filed by the Company pursuant to Sections
13(a),  13(c),  14 and 15(d) of the Exchange Act  subsequent to the date of this
Registration  Statement  and prior to the filing of a  post-effective  amendment
that  indicates  that all  securities  offered  herein  have been sold,  or that
deregisters  all such securities  then remaining  unsold,  shall be deemed to be
incorporated by reference in this Registration Statement and to be a part hereof
from the respective dates of filing of such documents.

Item 4.  Description of Securities.

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.

     The  validity of the Common  Stock  offered  hereby has been passed upon by
Seth H.  Hoogasian,  Esq.,  General Counsel of the Company.  Mr.  Hoogasian is a
full-time  employee of Thermo  Electron  Corporation  ("Thermo  Electron"),  the
majority  stockholder  of the  Company,  is an officer of the Company and Thermo
Electron,  and owns or has the right to acquire  22,331  shares of Common Stock,
379,785  shares of the  common  stock,  $1.00 par  value  per  share,  of Thermo
Electron, and 20,833 shares of the common stock of the Company's subsidiaries.

Item 6.  Indemnification of Directors and Officers.

      The Delaware  General  Corporation  Law and the Company's  Certificate  of
Incorporation  and By-Laws  limit the  monetary  liability  of  directors to the
Company and to its stockholders and provide for indemnification of the Company's
officers and directors for  liabilities and expenses that they may incur in such
capacities.  In general,  officers and directors are indemnified with respect to
actions  taken in good faith in a manner  reasonably  believed  to be in, or not
opposed to, the best  interests of the Company and, with respect to any criminal
action or proceeding,  actions that the  indemnitee  had no reasonable  cause to
believe were unlawful. The Company also has indemnification  agreements with its
directors and officers that provide for the maximum  indemnification  allowed by
law.

      The Company has an  insurance  policy  which  insures  the  directors  and
officers of the Company and its subsidiaries  against certain  liabilities which
might be incurred in connection with the performance of their duties.

Item 7.  Exemption from Registration Claimed.

      Not Applicable.
Item 8.  Exhibits.

      The Exhibit Index  immediately  preceding the exhibits is attached  hereto
and incorporated herein by reference.

Item 9.  Undertakings.

      (a)   The undersigned Registrant hereby undertakes:

            (1) To file,  during any  period in which  offers or sales are being
made, a post-effective amendment to this registration statement:

                  (i)    To include any prospectus required by Section 10(a)(3)
                           of the Securities Act of 1933;

                  (ii) To reflect in the  prospectus any facts or events arising
      after the effective date of the registration statement (or the most recent
      post-effective amendment thereof) which, individually or in the aggregate,
      represent  a  fundamental  change  in the  information  set  forth  in the
      registration  statement.  Notwithstanding  the foregoing,  any increase or
      decrease in volume of  securities  offered (if the total  dollar  value of
      securities  offered  would not exceed that which was  registered)  and any
      deviation from the low or high end of the estimated maximum offering range
      may be  reflected  in the form of  prospectus  filed  with the  Commission
      pursuant  to Rule 424(b) if, in the  aggregate,  the changes in volume and
      price  represent no more than 20 percent  change in the maximum  aggregate
      offering price set forth in the "Calculation of Registration Fee" table in
      the effective registration statement;

                  (iii) To include any material  information with respect to the
      plan  of  distribution  not  previously   disclosed  in  the  registration
      statement or any material change to such  information in the  registration
      statement.

      Provided,  however,  that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply
if the  information  required to be included in a  post-effective  amendment  by
those paragraphs is contained in periodic reports filed with or furnished to the
Commission  by the  Registrant  pursuant  to Section 13 or Section  15(d) of the
Securities  Exchange  Act of 1934  that are  incorporated  by  reference  in the
registration statement.

            (2) That,  for the purpose of  determining  any liability  under the
Securities Act of 1933, each such post-effective amendment shall be deemed to be
a new registration statement relating to the securities offered therein, and the
offering of such  securities at that time shall be deemed to be the initial bona
fide offering thereof.

            (3)  To  remove  from  registration  by  means  of a  post-effective
amendment  any of the  securities  being  registered  which remain unsold at the
termination of the offering.
(b) The undersigned  Registrant  hereby  undertakes  that, for purposes of
determining  any liability  under the Securities Act of 1933, each filing of the
Registrant's  annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable,  each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the  registration  statement shall be
deemed to be a new  registration  statement  relating to the securities  offered
therein,  and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.

      (c)  Insofar  as  indemnification   for  liabilities   arising  under  the
Securities Act of 1933 may be permitted to directors,  officers and  controlling
persons of the Registrant  pursuant to the foregoing  provisions,  or otherwise,
the  Registrant  has been  advised  that in the  opinion of the  Securities  and
Exchange  Commission such  indemnification is against public policy as expressed
in the Act and is,  therefore,  unenforceable.  In the  event  that a claim  for
indemnification  against  such  liabilities  (other  than  the  payment  by  the
Registrant of expenses  incurred or paid by a director,  officer or  controlling
person of the  Registrant  in the  successful  defense  of any  action,  suit or
proceeding)  is  asserted by such  director,  officer or  controlling  person in
connection with the securities being registered,  the Registrant will, unless in
the opinion of its counsel the matter has been settled by controlling precedent,
submit  to a  court  of  appropriate  jurisdiction  the  question  whether  such
indemnification  by it is against public policy as expressed in the Act and will
be governed by the final adjudication of such issue.
SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies  that it has  reasonable  grounds to believe  that it meets all of the
requirements  for  filing  on Form S-8 and has  duly  caused  this  Registration
Statement  to be  signed  on its  behalf  by  the  undersigned,  thereunto  duly
authorized, in the City of Waltham,  Commonwealth of Massachusetts,  on this 9th
day of February, 2000.

                                   THERMO INSTRUMENT SYSTEMS INC.

                                   By:  /s/ Earl R. Lewis
                                        ----------------------------------
                                        Earl R. Lewis
                                        President and Chief Executive
                                        Officer

                                POWER OF ATTORNEY

      Each of the  undersigned  Directors  and  Officers  of  Thermo  Instrument
Systems Inc. hereby appoints Theo  Melas-Kyriazi,  Paul F. Kelleher,  Kenneth J.
Apicerno,  Seth H.  Hoogasian and Sandra L. Lambert,  and each of them, his true
and lawful  attorneys-in-fact  and agents, with full power of substitution,  for
him and in his name, place and stead, in any and all capacities, to sign any and
all  amendments  (including  post-effective  amendments)  to  this  Registration
Statement, and to file the same, with all exhibits thereto, and all documents in
connection therewith, with the Securities and Exchange Commission, granting unto
said attorneys-in-fact and agents, and each of them, full power and authority to
do and perform each and every act and thing  requisite  and necessary to be done
in and about the  premises,  as fully to all intents and purposes as he might or
could  do  in  person,   hereby   ratifying   and   confirming   all  that  said
attorneys-in-fact  and  agents  or any of them,  or their or his  substitute  or
substitutes, may lawfully do or cause to be done by virtue thereof.

      Pursuant  to  the  requirements  of  the  Securities  Act  of  1933,  this
Registration  Statement  has been signed below by the  following  persons in the
capacities and on the dates indicated:

 Signature                           Title                       Date

/s/ Earl R. Lewis             President, Chief Executive    February 9, 2000
-------------------------     Officer and Director
Earl R. Lewis
/s/ Theo Melas-Kyriazi        Chief Financial Officer       February 9, 2000
-------------------------
Theo Melas-Kyriazi

/s/ Paul F. Kelleher          Chief Accounting Officer      February 9, 2000
-------------------------
Paul F. Kelleher

/s/ Arvin H. Smith            Chairman of the Board         February 9, 2000
-------------------------
Arvin H. Smith

/s/ George N. Hatsopoulos     Director                      February 9, 2000
-------------------------
George N. Hatsopoulos

/s/ John N. Hatsopoulos       Director                      February 9, 2000
-------------------------
John N. Hatsopoulos

/s/ Richard F. Syron          Director                      February 9, 2000
-------------------------
Richard F. Syron

/s/ Polyvios C. Vintiadis     Director                      February 9, 2000
-------------------------
Polyvios C. Vintiadis
EXHIBIT INDEX

Exhibit
Number            Description

5                 Opinion of Seth H. Hoogasian, Esq.

23.1              Consent of Arthur Andersen LLP

23.2              Consent of Seth H. Hoogasian, Esq.
                  (contained in his opinion filed as
                  Exhibit 5).

24                Power of Attorney (see  signature  pages to this  Registration
                  Statement).
EXHIBIT 5

                         Thermo Instrument Systems Inc.
                                 81 Wyman Street
                        Waltham, Massachusetts 02454-9046

                                                      February 9, 2000

Thermo Instrument Systems Inc.
81 Wyman Street
Waltham, Massachusetts 02454-9046

Re:   Registration Statement on Form S-8
      Relating to 324,416 Shares of the Common Stock,
      $.10 par value, of Thermo Instrument Systems Inc.

Dear Sirs:

      I am  General  Counsel  to Thermo  Instrument  Systems  Inc.,  a  Delaware
corporation  (the  "Company"),  and have acted as counsel in connection with the
registration  under the  Securities  Act of 1933,  as amended,  on Form S-8 (the
"Registration Statement"), of 324,416 shares of the Company's common stock, $.10
par value per share (the  "Shares")  subject to the  Thermo  Vision  Corporation
Equity  Incentive  Plan,  the  Thermo  Electron   Corporation  -  Thermo  Vision
Corporation Nonqualified Stock Option Plan, the Thermo Instrument Systems Inc. -
Thermo Vision  Corporation  Nonqualified  Stock Option Plan, the Thermo Electron
Corporation  Directors Stock Option Plan (Thermo Vision Shares),  and the Thermo
Instrument  Systems Inc.  Directors  Stock Option Plan  (Thermo  Vision  Shares)
(collectively, the "Plans").

      I or a member of my legal staff have  reviewed the  corporate  proceedings
taken by the Company  with respect to the  authorization  of the issuance of the
Shares.  I or a member of my legal  staff have also  examined  and  relied  upon
originals or copies, certified or otherwise authenticated to my satisfaction, of
all corporate records, documents, agreements or other instruments of the Company
and have made all  investigations  of law and have  discussed with the Company's
representatives   all  questions  of  fact  that  I  have  deemed  necessary  or
appropriate.
Based upon and subject to the foregoing, I am of the opinion that:

      1. The Company is a  corporation  validly  existing and in corporate  good
standing under the laws of the State of Delaware.

      2. The issuance and sale of the Shares as contemplated in the Registration
Statement have been duly authorized by the Company.

      3. The Shares,  when issued and sold in accordance  with the provisions of
the respective Plans, will be validly issued, fully paid and nonassessable.

      I hereby  consent  to the  filing  of this  opinion  as  Exhibit  5 to the
Registration Statement.

                                        Very truly yours,

                                        /s/ Seth H. Hoogasian
                                        ----------------------------
                                        Seth H. Hoogasian
                                        General Counsel
Exhibit 23.1

            Consent of Independent Public Accountants
            -----------------------------------------

     As independent public  accountants,  we hereby consent to the incorporation
by  reference  in this  registration  statement  on Form S-8 of our report dated
February 16, 1999  (except with respect to the matters  discussed in Note 18, as
to which  the date is March 1,  1999),  included  in Thermo  Instrument  Systems
Inc.'s Annual Report on Form 10-K for the year ended January 2, 1999, and to all
references to our Firm included in this registration statement.

                                             Arthur Andersen LLP

Boston, Massachusetts
February 4, 2000