<PAGE>   1
                                                                    EXHIBIT 4.3

                        FIRST AMENDMENT TO LOAN AGREEMENT

         This First Amendment to Loan Agreement is made as of the 21st day of
August, 1998 by and among

         Galileo Corporation, a Delaware corporation, with its principal place
         of business at Galileo Park, Sturbridge, Worcester County,
         Massachusetts (the "Borrower"); and

         BankBoston, N.A., a national banking association having its principal
         offices at 100 Federal Street, Boston, Massachusetts (the "Bank")

In consideration of the mutual covenants herein contained and benefits to be
derived herefrom.

                                   WITNESSETH

         WHEREAS, the Borrower and the Bank have entered into a Loan Agreement
dated as of January 27, 1998 (the "Loan Agreement"); and

         WHEREAS, certain Events of Default have arisen under the Loan Agreement
and the Borrower has requested the Bank to amend the Loan Agreement and to
forbear from exercising its rights and remedies as a result of the existence of
such Events of Default; and

         WHEREAS, the Bank is willing to forbear from exercising its rights and
remedies as a result of the existence of the Events of Default and to amend the
Loan Agreement on the terms set forth herein.

         NOW THEREFORE, it is hereby agreed as follows:

         1. Definitions: All capitalized terms used herein and not otherwise
defined shall have the same meaning herein as the Loan Agreement.

         2. Amendment to Section 1. The provisions of section 1.1 of the Loan
Agreement are hereby amended

                  (a) By deleting the definitions of "Adjusted LIBOR Rate",
"Conversion Date", "LIBOR Loan", "Interest Period" and "Notice of Borrowing
and/or Selection of Interest", in their entirety.

                  (b) By amending the definition of "Guarantor" by adding the
words "and Optical Filter Corporation" at the end thereof.

                  (c) By amending the definition of "Guaranty" by adding the
words "and the Guaranty of Optical Filter Corporation dated January 30, 1998" at
the end thereof.

                  (d) By amending the definition of "landlord's Waiver" by
deleting the words "at any location in which the value of the Borrower's assets
or any of its Subsidiaries exceed ONE MILLION and 00/100 ($1,000,000.00)
DOLLARS" and substituting in its stead the words "at any location at which
assets of the Borrower or any of its Subsidiaries are maintained".



<PAGE>   2
                  (e) By amending the definition of "Maximum Commitment" to read
"FOURTEEN MILLION and 00/100 DOLLARS ($14,000,000.00), subject to reduction as
provided in Section 2.1(b) hereof.

                  (f) By amending the definition of "Revolving Credit Period" by
deleting the words "or the Conversion Date" and substituting in its stead the
words "or the Maturity Date".

                  (g) By deleting the definition of "Security Agreements" in its
entirety and substituting the following in its stead:

                           Security Agreements. A certain Security Agreement of
                           even date herewith by and between the BORROWER and
                           the BANK encumbering the assets of the BORROWER, the
                           Security Agreements now or hereafter executed and
                           delivered by the BORROWER'S Subsidiaries to the BANK,
                           and all other instruments, documents and agreements
                           heretofore or hereafter executed and delivered by the
                           BORROWER or any of its Subsidiaries to the BANK
                           granting a Lien on any of their assets as security
                           for the Obligations.

                  (h) By adding the following new definitions:

                           Adjusted Interest Coverage Ratio. At any time of
                           determination, the ratio of (x) Consolidated
                           Operating Cash Flow plus or minus Consolidated
                           Working Capital Changes, as applicable, to (y)
                           Consolidated Interest Expense.

                           Asset Sale. Any one or series of related transactions
                           in which the Borrower or any of its Subsidiaries
                           conveys, sells, transfers or otherwise disposes of,
                           directly or indirectly, any of its properties,
                           business or assets (including the sale or issuance of
                           any capital stock of its respective Subsidiaries) or
                           merges or consolidates with or into, or enters into
                           any joint venture or other business combination with,
                           any other person or entity.

                           Consolidated Current Assets. All assets of the
                           BORROWER and its Subsidiaries on a consolidated basis
                           that, in accordance with generally accepted
                           accounting principles, are properly classified as
                           current assets.

                           Consolidated Current Liabilities. All liabilities and
                           other Indebtedness of the BORROWER and its
                           Subsidiaries on a consolidated basis maturing on
                           demand or within one (1) year from the date
                           Consolidated Current Liabilities are to be
                           determined, and such other liabilities that, in
                           accordance with generally accepted accounting
                           principles, are properly classified as current
                           liabilities.

                           Consolidated EBITDA. For any period, an amount equal
                           to Consolidated Net Income for such period, plus, to
                           the extent deducted in the calculation of
                           Consolidated Net Income for such period (a)
                           depreciation, amortization 


<PAGE>   3
                           and other noncash expenses, (b) income tax expense,
                           and (c) Consolidated Interest Expense, all as
                           determined in accordance with generally accepted
                           accounting principles.

                           Consolidated Interest Expense. For any period, the
                           aggregate amount of interest required to be paid or
                           accrued by the BORROWER and its Subsidiaries during
                           such period on all Indebtedness, including all
                           facility and other fees incurred in connection
                           therewith.

                           Consolidated Operating Cash Flow. For any period, an
                           amount equal to (a) Consolidated EBITDA for such
                           period, less (b) the sum of (I) cash payments for all
                           taxes made during such period, and (ii) to the extent
                           not already deducted in the determination of
                           Consolidated EBITDA, capital expenditures made during
                           such period to the extent permitted under Section
                           5.11 hereof.

                           Consolidated Working Capital Changes. For any period,
                           the net change from the immediately preceding period
                           to (a) accounts receivable of the BORROWER and its
                           Subsidiaries, (b) accounts payable of the BORROWER
                           and its Subsidiaries, (c) current accruals of the
                           BORROWER and its Subsidiaries, and (d) inventory of
                           the BORROWER and its Subsidiaries, all as determined
                           in accordance in generally accepted accounting
                           principles.

                           Current Ratio. At any time of determination, the
                           ratio of Consolidated Current assets to Consolidated
                           Current Liabilities.

                           Interest Coverage Ratio. At any time of
                           determination, the ratio of (x) Consolidated
                           Operating Cash Flow to (y) Consolidated Interest
                           Expense.

                           Maturity Date. December 31, 1999.

                           Net Cash Proceeds. The net cash proceeds received by
                           the Borrower or any of its Subsidiaries in respect of
                           any Asset Sale, less the sum of (a) all reasonable
                           out-of-pocket fees, commissions and other expenses
                           incurred in connection with such Asset Sale (but
                           excluding any applicable taxes required to be paid as
                           a result of any such Asset sale), and (b) the
                           aggregate amount of cash which is used to retire (in
                           whole or in part) any Indebtedness secured by a Lien
                           permitted hereunder having priority over the Liens of
                           the Bank with respect to the assets which are the
                           subject of the Asset sale and which is required to be
                           repaid in whole or in part in connection with such
                           Asset Sale.

         3. Amendments to Section 2. The provisions of section 2 of the Loan
Agreement are hereby amended

                  (a) By deleting the provisions of section 2.1 in its entirety
and substituting the following in its stead:




<PAGE>   4
                           2.1 The Loan. (a) Subject to the terms and conditions
                           hereof, the BANK will make available to the BORROWER
                           Loans in the aggregate amount outstanding of up to
                           the Maximum Commitment evidenced by a FOURTEEN
                           MILLION and 00/100 DOLLAR note ("Note") with interest
                           payable monthly at the aggregate of the Base Rate
                           plus one percent (1%) per annum. During the revolving
                           Credit Period, the BORROWER may borrow, prepay, and
                           reborrow pursuant to this Agreement. The entire
                           outstanding balance of the Loan shall be paid in full
                           on the Maturity date.

                           (b) The Borrower shall make a prepayment of the
                           Obligations in an amount equal to 100% of the Net
                           Cash Proceeds received by the Borrower or any of its
                           Subsidiaries from any Asset Sale. Upon any such Asset
                           Sale, the Maximum Commitment will be reduced by a n
                           amount equal to such Net Cash Proceeds. Any reduction
                           of the Maximum Commitment may not be reinstated.

b. by deleting the provisions of Section 2.2(a) and substituting the following
in its stead:

                           a) Whenever the BORROWER desires to obtain a Loan
                           hereunder, the BORROWER shall notify the BANK (which
                           notice shall be irrevocable) by telex, telegraph or
                           telephone received no later than 10:00 a.m. (Boston,
                           MA time) on the date one (1) business day before the
                           day on which the requested Loan is to be made. Such
                           notice shall specify the effective date and amount of
                           each Loan. Each such notification shall be
                           immediately followed by a written confirmation
                           thereof by the BORROWER in substantially the form of
                           Exhibit "B" hereto, provided that if such written
                           confirmation differs in any material respect from the
                           action taken by the BANK, the records of the BANK
                           shall control, absent manifest error.

c. by deleting the provisions of Section 2.4 in their entirety.

d. by deleting the words "one-quarter (.25%)" in Section 2.5(d) and substituting
in its stead the words "one-half percent (.50%)".

e. by adding the following new subsection at the end of Section 2.5:

                           e) An amendment fee in connection with the First
                           Amendment to this Agreement in the sum of
                           $150,000.00. Such amendment fee shall be paid in two
                           installments, the first of which shall be in the sum
                           of $50,000.00 and shall be paid upon execution of the
                           First Amendment to this Agreement and the second of
                           which shall be in the sum of $100,000.00 and shall be
                           paid on January 2, 1999. The full amount of the
                           amendment fee shall be fully earned upon the
                           execution of the First Amendment to this Agreement
                           and, except as set forth in the following sentence,
                           shall not be subject to refund or rebate under any
                           circumstances. Notwithstanding the foregoing, if (x)
                           all Obligations have been irrevocably paid in full on


<PAGE>   5
                           or before December 31, 1998, and (y) all obligations
                           of the BANK to make Loans terminated on or before
                           December 31, 1998, and (z) no Event of Default has
                           arisen after the date of the First Amendment to this
                           Agreement as a result of which the BANK has
                           accelerated the time for payment of the Obligations
                           and commenced the exercise of its remedies upon
                           default, the BANK shall waive payment of the second
                           installment of the amendment fee due on January 2,
                           1999.

f. by deleting the provisions of Section 2.6 in their entirety and substituting
the following in its stead:

                           2.6 Interest Rate and Payments of Interest: Each Base
                           Rate Loan shall bear interest on the outstanding
                           principal amount thereof at a rate per annum equal to
                           the aggregate of the Base Rate plus one percent (1%),
                           which rate shall change contemporaneously with any
                           change in the Base Rate. Interest shall be payable
                           monthly in arrears on the first day of each month.

g. by deleting the provisions of Section 2.7(a) in their entirety.

h. by deleting the words "(other than such requirements as are already included
in the determination of Adjusted LIBOR Rate)" in clause (2) of Section 2.7(b).

i. by deleting the provisions of Section 2.8 in their entirely and substituting
the following in its stead:

                           2.8 Payments and Prepayments of the Loan. All Base
                           Rate Loans may be prepaid in increments of a minimum
                           of $100,000.00 at any time without premium or penalty
                           on one (1) business day's notice and the interest
                           accrued on the amount so paid to the date of such
                           payment must be paid at the time of any such payment.

j. by deleting the provisions of Section 2.1(a) and substituting the following
in its stead:

                           a) Upon the occurrence of any Event of Default,
                           interest shall accrue on the outstanding principal of
                           the Obligations and, to the extent permitted by
                           applicable law, overdue interest and fees or other
                           amounts payable hereunder or under the other Loan
                           Documents at a rate per annum equal to the aggregate
                           of the Base Rate plus four percent (4%).

k. by deleting the provisions of Section 2.11 in their entirety.


l. by deleting the words "Except as provided in Paragraph (a ) of the definition
of Interest Period" from the second sentence of Section 2.12.




<PAGE>   6
4. Amendments to Section 5. The provisions of Section 5 of the Loan Agreement
are hereby amended


a. by deleting the words "five (5) days" appearing in Section 5.1(g) and
substituting the words "two (2) days" in their stead.


b. by adding the following new subsection to Section 5.1:


                           j) promptly, and in any event no less frequently than
                           weekly, written notice of the status of any
                           negotiations, offers and/or agreements for any Asset
                           Sales, together with copies of all offers, agreements
                           and other documents in connection therewith.


c. by deleting the provisions of Section 5.8 in their entirety and substituting
the following in its stead:


                           5.8 Current Ratio. The BORROWER and its Subsidiaries
                           will maintain a Current Ratio of no less than the
                           following as of the end of each of the months
                           indicated:

<TABLE>
<CAPTION>
                               Month Ending                 Maximum Ratio
                               ------------                 -------------
<S>                                                         <C>    
                               September 30, 1998           1.20:1.00

                               October 31, 1998             1.20:1.00

                               November 30, 1998            1.00:1.00

                               December 31, 1998            1.20:1.00

                               December 31, 1998 and each   1.10:1.00
                               month end thereafter
</TABLE>

d. by deleting the provisions of Section 5.9 in their entirety and substituting
the following in its stead:




<PAGE>   7
                           5.9 Ratio of Indebtedness to Consolidated Tangible
                           Net Worth. The BORROWER and its Subsidiaries will
                           maintain a ratio of Indebtedness to Consolidated
                           Tangible Net Worth of no more than the following as
                           of the end of each of the months indicated:

<TABLE>
<CAPTION>
                               Month Ending                 Minimum Ratio
                               ------------                 -------------
<S>                                                         <C>    
                               September 30, 1998           1.00:1.00

                               October 31, 1998             1.00:1.00

                               November 30, 1998            1.00:1.00

                               December 31, 1998            1.00:1.00

                               January 31, 1999 and each    1.20:1.00
                               month end thereafter
</TABLE>

e, by deleting the provisions of Section 5.10 in their entirety and substituting
the following in its stead:

                           5.10 Interest Coverage Ratio. (a) The BORROWER and
                           its Subsidiaries shall achieve an Interest coverage
                           Ratio of no less than (I) 1.30:1.00 at December 31,
                           1998 (calculated for the three months beginning
                           October 1, 1998 and ending December 31, 1998), and
                           (ii) 1.50:1.00 for each month thereafter commencing
                           with the month ending January 31, 1999.

                           (b) The BORROWER and its Subsidiaries shall achieve
                           an Adjusted interest Coverage Ratio of no less than
                           (I) 1.30:1.00 at December 31, 1998 (calculated for
                           the three months beginning October 1, 1998 and ending
                           December 31, 1998), and (ii) 1.50:1.00 for each month
                           thereafter commencing with the month ending January
                           31, 1999.

         f.       by deleting the provisions of Section 5.11 in their entirety
                  and substituting the following in their stead:

                           5.11 Limitation on Capital Expenditures. The BORROWER
                           and its Subsidiaries shall not make or incur capital
                           expenditures in excess of $400,000.00 in the
                           aggregate in any fiscal quarter.




<PAGE>   8
         g.       by deleting the provisions of Section 5.13 in their entirety
                  and substituting the following in its stead:

                           5.13 Consolidated Net Income. (a) the BORROWER and
                           its Subsidiaries will not permit its consolidated net
                           loss (before taxes and excluding non-cash deductions
                           associated with the closure of the medical endoscope
                           business), determined in accordance with generally
                           accepted accounting principals, to be greater than
                           $2,000,000.00 for the quarter ending September 30,
                           1998.

                           (b) The BORROWER and its Subsidiaries shall achieve
                           Consolidated Net Income (before taxes and excluding
                           any extraordinary gains and extraordinary losses
                           which would otherwise be included in the calculation
                           of Consolidated Net Income), for the fiscal quarter
                           ending December 31, 1998 of at least $200,000.00

                           (c) The BORROWER and its Subsidiaries shall achieve
                           Consolidated Net Income (before taxes and excluding
                           any extraordinary gains which would otherwise be
                           included in the calculation of Consolidated Net
                           Income), for each fiscal quarter of at least
                           $200,000.00, commencing with the fiscal quarter
                           ending march 31, 1999.

         h.       by adding the following new sections to the Loan Agreement.

                           5.14 Disposition of Channeltron and Related Product
                           Lines. On or before October 1, 1998, the BORROWER
                           shall enter into an agreement, on terms and
                           conditions reasonably satisfactory to the BANK, for
                           the sale of Scientific Detector's channeltron
                           detector and assemblies product lines for a purchase
                           price of at least $4,500,000 (the assets of which
                           have a book value of less than $2,500,000); and the
                           BORROWER shall cause such sale to be consummated and
                           the net Cash Proceeds therefrom to have been paid to
                           the BANK on or before November 1, 1998.

                           5.15. Retention of Consultant. The BORROWER shall
                           engage The Recovery Group or another consultant
                           reasonably acceptable to the BANK to undertake such
                           activities and duties as the BANK may reasonably
                           require. The BORROWER and its Subsidiaries shall
                           cooperate with such consultant in the performance of
                           his duties and hereby agrees that the consultant may,
                           without any notice to, or further consent from, the
                           BORROWER or its Subsidiaries, disclose any and all
                           information regarding the BORROWER and its
                           Subsidiaries obtained by the consultant in the
                           performance of his duties to the BANK. All reasonable
                           fees, costs and expenses of the consultant shall be
                           borne by the BORROWER and its Subsidiaries.




<PAGE>   9
                           5.16 Loan to Value. The BORROWER shall not permit the
                           ratio of the outstanding Obligations to the orderly
                           liquidation value (as reasonably determined by the
                           BANK) of the Collateral to be greater than 0.70:1.00.

                           5.17 Proceeds of Collateral. The BORROWER and its
                           Subsidiaries have previously established a lock box
                           with the BANK. All proceeds of the Collateral shall
                           continue to be directed to the lockbox, and shall be
                           deposited into the BORROWER'S operating account
                           maintained with the BANK until the earlier of
                           December 1, 1998 or the occurrence of an Event of
                           Default, at which time the BANK may apply the
                           proceeds in the lockbox to the Obligations.

                           5.18 Additional Information. The BORROWER and its
                           Subsidiaries shall cooperate with the BANK, the
                           consultant and their respective representatives in
                           order that the BANK shall receive a commercial
                           finance examination, a real estate appraisal, an
                           inventory valuation, and a final report of The
                           Recovery Group, each in form and substance
                           satisfactory to the BANK by September 13, 1998.

         5. Amendments to Section 6. The provisions of Section 6 of the Loan
Agreement are hereby amended

         a.       by adding the words "and its Subsidiaries" in Sections 6.2,
                  6.2, 6.4, and 6.5 after the words "the BORROWER" in the first
                  line of each of such sections.

         b.       by deleting the provisions of Section 6.1(e) and 6.1(f) in
                  their entirety.

         c.       by deleting the provisions of Section 6.2(f) in its entirety.

         d.       by deleting the words "FIVE HUNDRED THOUSAND AND 00/100
                  DOLLARS ($500,000.00)" in clause (ii) of Section 6.4 and
                  substituting the words "TWO HUNDRED THOUSAND AND 00/100
                  DOLLARS ($200,000.00) in its stead.

         e.       by deleting the provisions of Section 6.4(iii) in their
                  entirety.

         f.       by adding the following new clauses at the end of Section 6.4:

                           and (vi) the sale or other disposition of Scientific
                           Detector's channeltron detector and assemblies
                           product lines in accordance with the provisions of
                           Section 5.14 hereof.

         6. Amendments to Section 7. The provisions of Section 7 of the Loan
Agreement are hereby amended

         a.       by deleting clause m) in its entirety.

         b.       by adding the following new clause thereto:




<PAGE>   10
                           n) the occurrence of any default or event of default
under any other instrument, document, or agreement between the BORROWER or any
of its Subsidiaries and the BANK or any affiliate of the BANK.

         c.       by adding the following at the end thereof:

                           In addition, and without limiting any other remedies
of the BANK, upon the occurrence of any Event of Default, the BANK may institute
the lock box, clocked account or other procedures described in Section 5.17
hereof.

         7. Amendments to Section 8. The provision of Section 8.1 of the Loan
Agreement are hereby amended by changing the notice address for the BANK as
follows:

                  BankBoston, N.A.
                  100 Federal Street
                  Boston, Massachusetts 02110
                  Attention:  Ms. Corinne M. Barrett
                                    Mail Stop 01-06-01

                  Telephone:        (617) 434-0946
                  Telecopier:       (617) 434-4775

                  with a copy to:

                  Riemer & Braunstein
                  Three Center Plaza
                  Boston, Massachusetts  02108
                  Attention:  David S. Berman, Esquire

                  Telephone:        (617) 523-9000
                  Telecopier:       (617) 723-6831

         8. Amendments. Exhibits B and C to the Loan Agreement are hereby
deleted in their entirety and Exhibits B and C annexed hereto are substituted in
their stead.

         9. Conditions to Effectiveness. This First Amendment to Loan Agreement
shall not be effective until each of the following conditions precedent have
been fulfilled to the satisfaction of the BANK:

                  a. This First Amendment to the Loan Agreement shall have been
duly executed and delivered by the BORROWER, the Guarantors and the BANK, and
shall be in full force and effect. The BANK shall have received a fully executed
copy hereof and of each other document required hereunder.

                  b. All action on the part of the BORROWER and the Guarantors
necessary for the valid execution, delivery and performance by the BORROWER of
this First Amendment to Loan Agreement shall have been duly and effectively
taken. The BANK shall have received from each of the BORROWER and Guarantors,
true copies of their respective certificates of the 


<PAGE>   11
resolutions adopted by their respective boards of directors authorizing the
transactions described herein, each certified by their respective secretaries as
of a recent date to be true and complete.

         c. The BANK shall have received a list of all customers of the BORROWER
and its Subsidiaries (including, without limitation, all names, addresses,
contact persons and telephone numbers).

         d. The BANK shall have received (i) a duly executed Mortgage, Security
Agreement and Assignment of Leases and Rents (the "Mortgage) with respect to the
BORROWER'S real estate in Sturbridge, Massachusetts, evidence that such Mortgage
has been duly recorded and perfected and is subject to no Liens other than
Permitted Liens, and title insurance with respect to such Mortgage, (ii) duly
executed Collateral Assignments of Trademarks and Patents which shall have been
forwarded for filing with the United States Patent and Trademark Office, (iii) a
stock pledge with respect to the capital stock of each of the BORROWER'S
Subsidiaries (other than foreign Subsidiaries as to which the pledge will be
limited to 66% of such stock), (iv) duly executed guarantees of the Obligations
and Security Agreements from each of the BORROWER'S Subsidiaries (other than
foreign Subsidiaries) and (v) all Landlord's Waivers, all of the foregoing to be
in form and substance reasonably satisfactory to the BANK.

         e. The BORROWER and its Subsidiaries shall have executed security
agreements, collateral assignments, mortgages and such other documentation as
the BANK may require in order that all other obligations of the BORROWER and its
Subsidiaries to the BANK and to any affiliate of the BANK are secured by the
same Collateral as the Obligations under the Loan Agreement.

         f. The BANK shall have received opinions of counsel to each of the
BORROWER and Guarantors satisfactory to the BANK and the BANK's counsel.

         g. The BANK shall have been paid the initial installment of the
amendment fee in the sum of $50,000.00.

         h. The BORROWER shall have paid to the BANK all other fees and expenses
then due and owing pursuant to the Loan Agreement, as modified hereby, including
without limitation, reasonable attorneys' fees incurred by the BANK.

         i. No Default or Event of Default shall have occurred and be
continuing.

         j. The Borrowers and Guarantors shall have provided such additional
instruments and documents to the BANK as the BANK and its counsel may have
reasonably requested.

10. Miscellaneous.

         a. Except as provided herein, all terms and conditions of the Loan
Agreement and the other Loan Documents remain in full force and effect. The
BORROWER and the Guarantors hereby ratify, confirm, and reaffirm all of the
representations, warranties and covenants therein contained (except to the
extent that such representations and warranties expressly relate to an earlier
date). The BORROWER and the Guarantors further acknowledge and agree that none
of them have any offsets, defenses, or counterclaims against the BANK under the
Loan agreement 



<PAGE>   12
or the other Loan Documents and, to the extent that the BORROWER or the
Guarantors have, or ever had, any such offsets, defenses, or counterclaims, the
BORROWER and the Guarantors each hereby waive and release the same.

         b. The BANK hereby agrees to forbear from exercising its rights and
remedies as a result of the existence of any Defaults or Events of Default
existing prior to the date hereof under Sections 5.8, 5.9, 5.10 and 5.13 of the
Loan Agreement until the earlier of the Maturity Date or the occurrence of any
additional Event of Default under the Loan Agreement. This agreement to forebear
is not a waiver of any Defaults or Events of Default which hereafter arise under
such Sections of the Loan Agreement, as amended hereby.

         c. The BORROWER shall pay all costs and expenses incurred by the BANK
in connection with this First Amendment, including without limitation, all
reasonable attorneys' fees and expenses, commercial finance examination fees,
appraisal fees, consultant's fees, and all reasonable travel expenses incurred
by the BANK.

         d. This First Amendment may be executed in several counterparts and by
each party on a separate counterpart, each of which when so executed and
delivered, each shall be original, and all of which together shall constitute
one instrument.

         e. This First Amendment expresses the entire understanding of the
parties with respect to the matters set forth herein and supersedes all prior
discussions or negotiations hereon.

         IN WITNESS WHEREOF, the parties hereto have caused this First Amendment
to be executed and their seals to be hereto affixed as the date first above
written.

                                   "BORROWER"

                                   GALILEO CORPORATION


                                   By:__________________________________________
                                        Name:
                                        Title:

                                   "GUARANTORS"

                                   LEISEGANG MEDICAL, INC.


                                   By:__________________________________________
                                        Name:
                                        Title:

                                   "OPTICAL FILTER CORPORATION"


                                   By:__________________________________________
                                        Name:



<PAGE>   13
                                        Title:

                                   "BANK"

                                   BANKBOSTON, N.A.


                                   By:__________________________________________
                                        Name:
                                        Title: