<PAGE>   1
                                                                    EXHIBIT 10.6

                       SECOND AMENDMENT TO LOAN AGREEMENT

         This Second Amendment to Loan Agreement is made as of the 25th day of
January, 1999 by and between

         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, as amended by a First Amendment to Loan Agreement
dated as of August 21, 1998 (as amended and in effect, 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 in 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 definition of "Adjusted Interest Coverage
Ratio" in its entirety.

       b.       by deleting the definition of "Current Ratio" in its entirety 
and substituting the following in its stead:

                           CURRENT RATIO. At any time of determination, the
                           ratio of (x) Consolidated Current Assets to (y)
                           Consolidated Current Liabilities (exclusive of such
                           portion of the Obligations as may be properly
                           classified as current liabilities in accordance with
                           generally accepted accounting principles).

       c.         by amending the definition of "Indebtedness" by deleting the 
words "other than accounts payable and accrued expenses arising in the ordinary
course of business" appearing in clause (i) of such definition and substituting
the words "including accounts payable and accrued expenses arising in the
ordinary course of business" in their stead.

       d.         by amending the definition of "Maximum Commitment" to read 
"THIRTEEN MILLION and 00/100 DOLLARS ($13,000,000.00) through June 30, 1999, and
SIX MILLION and 00/100 DOLLARS ($6,000,000.00) from and after July 1, 1999,
subject to reduction in each case as provided in Section 2.1(b) hereof."


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<PAGE>   2

       e.         by amending the definition of "Maturity Date" to read
"October 31, 2000".

       f.         by adding the following new definitions:

                           APPROVED SALE. The sale of the Sturbridge Property
                           for a purchase price and on terms substantially as
                           set forth in the offer dated December 8, 1998 from
                           IRE-POLUS Group.

                           OCF II. At any time of determination, an amount equal
                           to (i) Consolidated EBITDA, PLUS OR MINUS (ii)
                           Consolidated Working Capital Changes, MINUS (iii)
                           cash payments for all income taxes made during such
                           period MINUS (iv) capital expenditures made, and paid
                           in cash, by the BORROWER during such period, all as
                           determined in accordance with generally accepted
                           accounting principles.

                           STURBRIDGE PROPERTY. The real property owned by the
                           Borrower and known as Galileo Park, Sturbridge,
                           Massachusetts and ancillary parcels thereto and the
                           telecommunications assets of the Borrower.

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:

                           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 THIRTEEN
                           MILLION and 00/100 DOLLAR note ("Note") with interest
                           payable monthly at the aggregate of the Base Rate
                           plus two percent (2%) 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, subject to the final sentence of this Section
                           2.1(b), the Maximum Commitment will be reduced by an
                           amount equal to such Net Cash Proceeds. Any reduction
                           of the Maximum Commitment may not be reinstated.
                           Notwithstanding the foregoing,

                           (i) provided that no Event of Default then exists or
would arise therefrom, the Borrower shall not be obligated to reduce the Maximum
Commitment by Five Hundred Thousand Dollars ($500,000.00) of Net Cash Proceeds
from the sale of the Sturbridge Property in an Approved Sale (the Maximum
Commitment being reduced by all Net Cash Proceeds in excess of such amount); and

                           (ii) The Maximum Commitment of $6,000,000.00 
effective from and after July 1, 1999 shall be reduced by Net Cash Proceeds from
Asset Sales occurring prior to that date only to the extent that such Net Cash
Proceeds, together with any Maximum Commitment reductions from other sources
prior to July 1, 1999, exceeded $7,000,000.00 in the aggregate.

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

                           f) An amendment fee in connection with the Second
                           Amendment to this Agreement in the sum of
                           $200,000.00. Such amendment fee shall be paid in 


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<PAGE>   3

                           four installments, each in the sum of $50,000.00,
                           payable quarterly, with the first such payment to be
                           made within seven (7) days after the effective date
                           of the Second Amendment to this Agreement and the
                           additional installments payable on April 30, 1999,
                           July 31, 1999, and October 31, 1999. The full amount
                           of the amendment fee shall be fully earned upon the
                           execution of the Second 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, (1) if
                           (x) all Obligations have been reduced to SIX MILLION
                           DOLLARS ($6,000,000.00) on or before July 1, 1999,
                           and (y) the ratio of the outstanding Obligations to
                           the orderly liquidation value (as reasonably
                           determined by the BANK) of the Collateral is not
                           greater than 0.60:1.00. as of July 1, 1999 and/or
                           October 1, 1999, and (z) no Event of Default has
                           arisen after the date of the Second 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 required amendment fee due in connection
                           with the Second Amendment on July 31, 1999 and/or
                           October 31, 1999 (or either of those dates if the
                           foregoing conditions are met on one but not both
                           dates) shall be reduced to $25,000.00, or (2) (A) if
                           no Event of Default has arisen after the date of the
                           Second 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 and (B) all Obligations are
                           paid in full before July 1, 1999, and (C) all
                           obligations to BancBoston Leasing Inc. under a Master
                           Lease Agreement dated March 20, 1998 and all
                           Schedules thereto are paid in full before July 1,
                           1999, the BANK will waive payment of the installments
                           of the amendment fee due on July 31, 1999 and October
                           31, 1999.

       c.         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 two percent (2%),
                           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.

4.       AMENDMENTS TO SECTION 5. The provisions of Section 5 of the Loan
         Agreement are hereby amended

       a.         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
                           1.50:1.00 as of the end of each month.

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

                                    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 quarters indicated:

                                      ------------------   -------------
                                      Quarter Ending       Maximum Ratio
                                      ------------------   -------------
                                      March 31, 1999       2.00:1.00
                                      ------------------   -------------


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<PAGE>   4
                                      ------------------   -------------
                                      June 30, 1999        1.50:1.00
                                      ------------------   -------------
                                      September 30, 1999   1.25:1.00
                                      ------------------   -------------
                                      December 31, 1999    1.25:1.00
                                      ------------------   -------------
                                      March 31, 2000       1.25:1.00
                                      ------------------   -------------
                                      June 30, 2000        1.25:1.00
                                      ------------------   -------------
                                      September 30, 2000   1.25:1.00
                                      ------------------   -------------
                                                         



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

                           5.10 INTEREST COVERAGE RATIO. The BORROWER and its
                           Subsidiaries shall achieve an Interest Coverage Ratio
                           of at least (i) 1.25:1.00 for the fiscal quarter
                           ending June 30, 1999, and (ii) 3.00:1.00 for each
                           fiscal quarter thereafter beginning with the fiscal
                           quarter ending September 30, 1999.

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

                           5.11 LIMITATIONS ON CAPITAL EXPENDITURES. The
                           BORROWER and its Subsidiaries shall not make or incur
                           capital expenditures in excess of the following
                           amounts in the aggregate in the following fiscal
                           quarters:

                                      ------------------    ---------------
                                      Quarter Ending        Maximum Capital
                                                            Expenditures
                                      ------------------    ---------------
                                      March 31, 1999        $1,340,000.00
                                      ------------------    ---------------
                                      June 30, 1999         $1,570,000.00
                                      ------------------    ---------------
                                      September 30, 1999    $1,504,000.00
                                      ------------------    ---------------
                                      December 31, 1999     $  775,000.00
                                      ------------------    ---------------
                                      March 31, 2000        $  605,000.00
                                      ------------------    ---------------
                                      June 30, 2000         $  635,000.00
                                      ------------------    ---------------
                                      September 30, 2000    $  635,000.00
                                      ------------------    ---------------
                                                        

       e.         by deleting the provisions of Section 5.13 in their entirety 
and substituting the following in its stead:

                           5.13     CONSOLIDATED NET INCOME. 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
                                    each month commencing with the month ending
                                    January 31, 1999 equal to or greater than
                                    the amounts set forth below:


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<PAGE>   5



<TABLE>
                           -----------------------------   -------------------------------------------------
                           <S>                             <C>            
                           Month Ending                    Minimum Consolidated Net Income "( )" means loss
                           -----------------------------   -------------------------------------------------
                           January 31, 1999                ($200,000.00)
                           -----------------------------   -------------------------------------------------
                           February 28, 1999               ($200,000.00)
                           -----------------------------   -------------------------------------------------
                           March 31, 1999                  ($100,000.00)
                           -----------------------------   -------------------------------------------------
                           April 30, 1999 and each month   
                           end thereafter                  $1.00
                           -----------------------------   -------------------------------------------------
                                                            
</TABLE>


       f.         by deleting the provisions of Section 5.14 in their entirety.

       g.         by deleting the provisions of Section 5.17 in their entirety 
and substituting the following in its stead:

                           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 occurrence of an
                           Event of Default, at which time the BANK may apply
                           the proceeds in the lockbox daily to the Obligations.

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

                           5.19 COMMERCIAL FINANCE EXAMS/APPRAISAL. The BORROWER
                           will cooperate with the BANK and permit the BANK to
                           undertake (at the expense of the BORROWER) commercial
                           finance examinations at such times as the BANK
                           determines; provided that the BORROWER shall not be
                           responsible for the cost of more than one commercial
                           finance examinations in any four (4) month period
                           unless an Event of Default exists (in which event the
                           BORROWER shall be responsible for all such expenses
                           incurred by the BANK). If the Sturbridge Property has
                           not been sold as of July 1, 1999, the BANK may
                           arrange for an appraisal of such property at the
                           BORROWER'S expense. All commercial finance exams and
                           appraisals shall be in form and substance
                           satisfactory to the BANK.

                           5.20 OCF II. The BORROWER and its Subsidiaries shall
                           achieve OCF II for each of the following fiscal
                           quarters equal to or greater than the following
                           amounts:
<TABLE>

                              ------------------------------------    ---------------
                              <S>                                     <C>
                              Fiscal Quarter Ending                   Minimum OCF II
                              ------------------------------------    ---------------
                              ------------------------------------    ---------------
                              March 31, 1999                          ($3,750,000.00)
                              ------------------------------------    ---------------
                              ------------------------------------    ---------------
                              June 30, 1999                           ($1,750,000.00)
                              ------------------------------------    ---------------
                              ------------------------------------    ---------------
                              September 30, 1999                       $200,000.00
                              ------------------------------------    ---------------
                              ------------------------------------    ---------------
                              December 31, 1999 and each month end    
                              thereafter                               $500,000.00
                              ------------------------------------    ---------------
</TABLE>


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<PAGE>   6

5.       AMENDMENTS TO SECTION 6.   The provisions of Section 6 of the Loan 
         Agreement are hereby amended

       a.         by adding the following clause to Section 6.1:

                           g)   Capital Leases entered into by Optical Filter 
                           Corporation in an amount not to exceed $2,800,000.00
                           in the aggregate.

       b.         by deleting the provisions of Section 6.4(vi) in their 
                  entirety and substituting the following in their stead:

                           (vi) Sell the Sturbridge Property in the Approved
                           Sale (upon which sale and the receipt of the Net Cash
                           Proceeds to which the BANK is entitled hereunder, the
                           BANK will release its mortgage liens on the
                           Sturbridge Property so sold).

       c.         by  adding the following at the end of Section 6.5 of the Loan
                  Agreement:

                           (other than the sale and leaseback of a portion of
                           the Sturbridge Property in connection with the
                           Approved Sale on terms reasonably satisfactory to the
                           BANK).

       d.         by adding the following new section:

                  6.6 DIVIDENDS AND DISTRIBUTIONS. The Borrower shall not:
                  declare or pay any dividend on or in respect of any shares of
                  any class of capital stock of the Borrower; purchase, redeem
                  or otherwise retire any shares of any class of capital stock
                  of the Borrower, directly or indirectly by the Borrower,
                  through a Subsidiary or otherwise; return capital by the
                  Borrower to its shareholders; or any other distribution on or
                  in respect of any shares of any class of capital stock of the
                  Borrower.

6.       CONDITIONS TO EFFECTIVENESS. This Second 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 Second Amendment to 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 Second 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 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.



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<PAGE>   7

       c.      The BORROWER shall have received additional equity proceeds from
Andlinger Trust XIII LLC in the sum of at least $6,000,000.00 substantially on
the terms set forth in the December 22, 1998 Securities Purchase Agreement, a
copy of which has been furnished the BANK.

       d.      The Obligations due to the BANK shall have been reduced to no 
more than the principal sum of $13,000,000.00.

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

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

       h.      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.

7.     MISCELLANEOUS.

       a.      In the event that the BORROWER pays all Obligations in full, all
obligations to BancBoston Leasing Inc. under a Master Lease Agreement dated
March 20, 1998 and all Schedules thereto shall become immediately due and
payable (notwithstanding any contrary payment provisions in those documents) and
the BORROWER shall contemporaneously with such payment to the BANK repay all
obligations of BancBoston Leasing Inc. thereunder.

       b.      Except as provided herein, all terms and conditions of the Loan
Agreement and the other Loan Documents remain in full force and effect. All
collateral previously or hereafter granted to secure the Obligations and the
Guaranties shall continue to secure the Obligations, as amended by this Second
Amendment to Loan Agreement. 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 or as set forth on Schedule I attached
hereto). 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 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.

       c.      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 4.14, 5.1, 5.4, 5.8, 5.10,
5.11, 5.13, 5.14 and 7(n) (only insofar as such section relates to defaults
under Paragraphs 14, 16.1(d)(insofar as it relates to Paragraphs 14 and 16.1(i)
thereof) and 16.1(i)(insofar as it relates to events of default under the Loan
Agreement) of a Master Lease Agreement dated March 20, 1998 between the BORROWER
and BancBoston Leasing Inc.) 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 forbear is not a waiver of any Defaults or
Events of Default which hereafter arise under such Sections of the Loan
Agreement, as amended 


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<PAGE>   8
hereby.

       d.      Within seven days after the effective date of this Amendment, the
BORROWER shall pay all costs and expenses incurred by the BANK in connection
with this Second 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.

       e.      This Second 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 an original, and all of which together shall constitute
one instrument.

       f.      This Second 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 Second
Amendment to be executed and their seals to be hereto affixed as the date first
above written.

                                              "BORROWER"
                                         GALILEO CORPORATION

                                         By: /s/ Josef W. Rokus     
                                             --------------------------
                                             Name: Josef W. Rokus
                                             Title: Vice President

                                         "GUARANTORS"
                                         LEISEGANG MEDICAL, INC.


                                         By: /s/ Josef W. Rokus     
                                             --------------------------
                                             Name: Josef W. Rokus
                                             Title: Secretary

                                         OPTICAL FILTER CORPORATION

                                         By: /s/ Josef W. Rokus     
                                             --------------------------
                                             Name: Josef W. Rokus
                                             Title: Secretary


                                         "BANK"
                                         BANKBOSTON, N.A.



                                         By: /s/ Corinne M. Barrett     
                                             --------------------------
                                             Name: Corinne M. Barrett
                                             Title: Vice President


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