Document:

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                                                                  EXHIBIT 10.8.1

                                             Confidential Treatment Requested by
                                              Avistar Communications Corporation

                MODIFICATION #5 TO PREFERRED SUPPLIER AGREEMENT
                       DATED JUNE 24, 1997 ("AGREEMENT").
       Effective Date of Modification June 27, 2000 ("Modification Date").

The parties hereby agree that as of the Modification Date, certain terms and
conditions of the Agreement, as expressly modified below, shall apply and that
this Modification #5 shall take precedence over the terms contained in the
Agreement and any prior modifications:

1.   NEW ORDER: In conjunction with execution of this Modification #5, an
     Avistar purchase order will be placed for an additional * (or, as
     applicable pursuant to Section 8, * Codec units) ("New Order"). The unit
     price for each Codec on the New Order shall be * plus a per unit
     maintenance fee of *, for a total of purchase price per unit of *. The per
     unit maintenance fee shall replace the annual maintenance fee set forth in
     the Agreement, Section 3.4(b).

2.   PAYMENT TERMS FOR NEW ORDER: Payment terms extended to Avistar for the New
     Order shall be strictly adhered to. Payment terms for all deliveries under
     the New Order shall be * from shipment and Avistar's receipt of invoice.

3.   DELIVERY OF NEW ORDER: Codec units will be shipped in accordance with the
     following estimated delivery schedule, as updated from time to time by
     Avistar. Unless otherwise mutually agreed in writing, Tandberg will ship
     all Codec units under this New Order by no later than *:

<TABLE>
<CAPTION>
# OF UNITS OF THE * TO BE SHIPPED         EXT. SHIP DATE
---------------------------------         --------------
<S>                                      <C>
*                                         *

</TABLE>

4.   MAINTENANCE AND WARRANTY FOR NEW ORDER: All Codec units delivered pursuant
     to this Modification #5 shall be subject to a * warranty and maintenance
     support period from shipment ("Term"). Maintenance services shall be as
     described in the Agreement, including all software upgrades (not deemed to
     be products in their own right) issued during the Term.

5.   ADDITIONAL ORDERS FOR MODEL * CODECS: Avistar acknowledges that Tandberg
     wishes to cease normal production cycles for the Tandberg * Codec and has
     provided Avistar a proposal for an enhanced replacement version as further
     described in Section 6 below. The parties agree that in the event Avistar
     wishes to continue purchasing additional * Codec units, Tandberg will
     manufacture them for Avistar until the end of the *, at the prices
     indicated in Section 1 above, provided that Avistar provides Tandberg with
     (i) a * lead-time from order placement to delivery; and (ii) a minimum
     order of * units.

6.   ENHANCED REPLACEMENT *: Tandberg has suggested it's * as alternative
     replacement Codecs to Avistar in place of the * codecs. The * will be
     available for delivery by no later than *. The functionality of the * shall
     have no less than the functionality and features of the * Codec at a price
     not to exceed * (including maintenance). A * will also be available to
     Avistar at a price not to exceed * (including maintenance). The parties
     will negotiate in good faith the prices for the Enhanced Codecs beginning
     in * based on order quantity, etc.

7.   TESTING UNITS: To facilitate the testing and development effort required to
     ensure Avistar's ability to use the * units proposed by Tandberg, Tandberg
     will, at Avistar's request, provide Avistar up to * loaner units of each
     model of Codec for development and testing purposes. These test units will
     either become part of the first order for * units, or will be returned to
     Tandberg by *. In the case of * units, the parties may mutually agree to
     (i) an extension to the loaner term for these units if Avistar is required
     to provide support for the * (ii) Avistar's return of * of it's existing
     * units in exchange for the * units.

8.   INTERIM SUBSTITUTION OF * CODEC: Tandberg may, at its discretion, ship *
     Codecs to fulfill this Order provided that (i) the * software is installed
     to ensure compatibility with the Avistar System and that the Codec is
     'packaged in such a way so as not to expressly indicate it is not a * unit.
     In the event *, Avistar will ensure that the * capabilities are not
     utilized without first paying Tandberg a * upgrade fee.

9.   ADDITIONAL PRODUCTS, PACKAGING AND SHIP-TO LOCATIONS: At Avistar's request,
     Tandberg agrees to package the Codec in a bundle to include certain
     additional hardware and cables consistent with the Bill-of-Material options
     attached hereto as Exhibit 1 ("Avistar BOM"). Certain Avistar BOM options
     include * units at * per unit. The prices for the additional components of
     the Avistar BOM are detailed on Exhibit 1. When ordered separately from
     Codec units, the * units must be ordered in minimum increments of * units.
     At no additional charge to Avistar, bundled shipments in minimum increments
     of * units, will be sent to one of the following two Avistar Locations, as
     specified by Avistar:

          US:   992 SAN ANTONIO ROAD, PALO ALTO -- CA
          ---------------------------------------------------------------
          UK:   TO BE PROVIDED
          ---------------------------------------------------------------

Except as expressly modified by this Modification #5, all terms and conditions
of the Agreement shall remain in full force and effect until December 31, 2001.
The parties agree that this Modification #5 may be executed via facsimile and
that it shall be considered equivalent to an executed original when signed (via
facsimile) by both parties; nevertheless, any one of the parties may require the
follow-up exchange of originals in hardcopy by so requesting in writing within
five (5) days of facsimile execution.

IN WITNESS WHEREOF, THE PARTIES HAVE CAUSED THIS MODIFICATION #5 TO BE EXECUTED
BY THEIR AUTHORIZED REPRESENTATIVES:

AVISTAR SYSTEMS CORPORATION                                        TANDBERG INC.

<TABLE>
<S>                                          <C>
By: /s/ RIMA VANHILL                         /s/ COLIN STURTEVANT
   --------------------------------------    -------------------------------------------
Printed Name/Title: Rima Vanhill,            Printed Name/Title: Colin Sturtevant
                    Director of Contracts                        Western Region Director
</TABLE>
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* Confidential portions omitted pursuant to a confidential treatment request
and filed separately with the Securities and Exchange Commission.<PAGE>   1
                                                                    Exhibit 10-A

                                 FIRST AMENDMENT
                                       TO
                      AMENDED AND RESTATED CREDIT AGREEMENT

         THIS FIRST AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (this
"Agreement"), dated as of March 31, 2000, is entered into among HERCULES
INCORPORATED, a Delaware corporation (the "Company"), BETZDEARBORN CANADA, INC.,
an Ontario corporation (the "Canadian Borrower") certain subsidiaries of the
Company identified on the signature pages hereto (the "Subsidiary Guarantors"),
BANK OF AMERICA, N.A., as Administrative Agent (the "Administrative Agent") for
and on behalf of the Lenders and BANK OF AMERICA CANADA, as Canadian
administrative agent (the "Canadian Administrative Agent"). Terms used but not
otherwise defined herein shall have the meanings provided in the Credit
Agreement described below.

                                    RECITALS

         A. The Company, the Canadian Borrower, the Subsidiary Guarantors, the
financial institutions party thereto (the "Lenders"), the Administrative Agent
and the Canadian Administrative Agent entered into that certain Credit Agreement
dated as of April 19, 1999 (the "Credit Agreement").

         B. The Company has requested certain modifications to the Credit
Agreement.

         C. Such modifications require the consent of the Required Lenders (as
defined in the Credit Agreement).

         D. The Required Lenders have consented to the requested modifications
on the terms and conditions set forth herein and have authorized the
Administrative Agent to enter into this Amendment on their behalf to give effect
to this Amendment.

                                    AGREEMENT

         NOW, THEREFORE, IN CONSIDERATION of the premises and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

         1. Amendment to Subsection 2.17. Subsection 2.17 of the Credit
Agreement is hereby amended as follows:

                  (a) Subsection 2.17(a) is amended by adding the following new
         paragraph to the end thereof:
<PAGE>   2
                           On the First Amendment Effective Date and in
                  accordance with the borrowing procedures set forth in
                  subsection (b) below, the Company shall convert some or all of
                  that portion of the Tranche A Term Loan then denominated in
                  Euros to Term Loans denominated in U.S. Dollars by requesting
                  a readvance in U.S. Dollars of such portion of the Tranche A
                  Term Loan and, upon receipt of such readvance, immediately
                  repaying to the Administrative Agent such portion of the
                  Tranche A Term Loan designated for conversion in Euros. The
                  Term Loans converted on the First Amendment Effective Date
                  pursuant to the preceding sentence shall continue to
                  constitute a portion of the Tranche A Term Loan.

                  (b) Subsection 2.17(b) is amended by adding the following new
         paragraph to the end thereof, immediately preceding subsection 2.17(c):

                           The conversion of a portion of the Tranche A Term
                  Loan on the First Amendment Effective Date is subject to the
                  following procedures: The Company shall submit an appropriate
                  notice of borrowing, substantially in the form of Exhibit G,
                  to the Administrative Agent not later than 11:00 A.M.
                  (Charlotte, North Carolina time) on the third Business Day
                  prior to the First Amendment Effective Date with respect to
                  the portion of the Tranche A Term Loan to be redenominated in
                  U.S. Dollars, which notice of borrowing shall be irrevocable
                  and shall specify (i) that a readvance in U.S. Dollars of a
                  portion of the Tranche A Term Loan is requested on the First
                  Amendment Effective Date, (ii) the amount in Euros of such
                  requested readvance (the "Conversion Readvance Amount") and
                  (iii) the requested Interest Period(s) for the Eurodollar
                  Loans comprising the Term Loans made in connection with such
                  readvance. The Administrative Agent will notify the Tranche A
                  Converting Lenders of such readvance request and will, by no
                  later than the second Business Day prior to the First
                  Amendment Effective Date, notify each Tranche A Converting
                  Lender of the amount of U.S. Dollars to be funded by such
                  Tranche A Converting Lender on the First Amendment Effective
                  Date in order to effectuate such readvance (each such amount a
                  "Tranche A Conversion Amount"). The amount of U.S. Dollars to
                  be funded by each Tranche A Converting Lender shall be
                  determined based on the Administrative Agent's spot rate of
                  exchange two Business Days prior to the First Amendment
                  Effective Date. Each Tranche A Converting Lender shall make
                  its Tranche A Conversion Amount available to the
                  Administrative Agent for the account of the Company at the
                  office of the Administrative Agent specified in Section 9.2,
                  or at such other address as the Administrative Agent may
                  specify to the Tranche A Converting Lenders and the Company,
                  by 11:00 a.m. (Charlotte, North Carolina time) on the First
                  Amendment Effective Date, in U.S. Dollars, and in funds
                  immediately available to the Administrative Agent. Upon
                  confirmation by the Administrative Agent of its receipt of a
                  Tranche A Converting Lender's Tranche A Conversion Amount in
                  U.S. Dollars, the Administrative Agent will, on the First
                  Amendment Effective Date, distribute to the Company such
                  Lender's Tranche A Conversion Amount in U.S. Dollars. On the
                  First Amendment Effective Date and immediately upon its
                  receipt of the respective Tranche A

                                       2
<PAGE>   3
                  Conversion Amounts referred to above, the Company shall pay to
                  the Administrative Agent, for the pro rata benefit of the
                  Tranche A Converting Lenders in Euros, the Conversion
                  Readvance Amount; provided, however, that if the
                  Administrative Agent has not made available to the Company any
                  Lender's Tranche A Conversion Amount in U.S. Dollars on the
                  First Amendment Effective Date, then the Company may withhold
                  from the amount otherwise payable to the Administrative Agent
                  pursuant to this sentence such Lender's pro rata portion of
                  the Conversion Readvance Amount until such time as such
                  Lender's Tranche A Conversion Amount has been made available
                  to the Company.

                  (c) Subsection 2.17 (d) is amended by deleting subclause (i)
         thereof and replacing it in its entirety with the following:

                           (i) The principal amount of the Tranche A Term Loan
                  shall be repaid on each of the Principal Amortization Payment
                  Dates set forth below with a quarterly payment on each such
                  date equal to an amount in U.S. Dollars equal to that portion
                  of the Tranche A Term Loan denominated in U.S. Dollars as of
                  the First Amendment Effective Date (as expressed in U.S.
                  Dollars) multiplied by the percentage set forth below
                  corresponding to the relevant Principal Amortization Payment
                  Date:

         2. Amendments to Section 7. Section 7 of the Credit Agreement is hereby
amended by adding the following new
definitions to Section 7:

                  "Conversion Readvance Amount" has the meaning set forth in
         subsection 2.17(b).

                  "First Amendment Effective Date" means March 31, 2000.

                  "Tranche A Conversion Amount" has the meaning set forth in
         subsection 2.17(b).

                  "Tranche A Converting Lender" means each Lender identified on
         Schedule I as having a Commitment to make Term Loans redenominated in
         U.S. Dollars under the Tranche A Term Loan, in its capacity as a Lender
         of such Term Loans, and each Lender that purchases an assignment of
         such a Commitment or such Loans in accordance with the terms of this
         Agreement.

         3. Amendment to Schedule I to Credit Agreement. Schedule I to the
Credit Agreement is hereby deleted in its entirety and replaced with Schedule I
as set forth in Annex I attached hereto.

         4. Representations and Warranties. Each of the Credit Parties
represents and warrants to the Lenders, the Administrative Agent and the
Canadian Administrative Agent as follows:

                  (a) It has taken all necessary action to authorize the
execution, delivery and performance of this Agreement.

                                       3
<PAGE>   4
                  (b) This Agreement has been duly executed and delivered by
         such Credit Party and constitutes such Credit Party's legal, valid and
         binding obligation, enforceable in accordance with its terms, except as
         such enforceability may be limited (x) by general principles of equity
         and conflicts of laws or (y) by bankruptcy, reorganization, insolvency,
         moratorium or other laws of general application relating to or
         affecting the enforcement, of creditors' rights.

                  (c) No consent, approval, authorization or order of, or
         filing, registration or qualification with, any court or governmental
         authority or third party is required in connection with the execution,
         delivery or performance by such Credit Party of this Agreement.

                  (d) The execution and delivery of this Agreement does not
         diminish or reduce its obligations under the Credit Documents
         (including, without limitation, in the case of each Guarantor, such
         Guarantor's guaranty pursuant to Section 3A of the Credit Agreement) in
         any manner, except as specifically set forth herein.

                  (e) Such Credit Party has no claims, counterclaims, offsets,
         or defenses to the Credit Documents and the performance of its
         obligations thereunder, or if such Credit Party has any such claims,
         counterclaims, offsets, or defenses to the Credit Documents or any
         transaction related to the Credit Documents, the same are hereby
         waived, relinquished and released in consideration of the Required
         Lenders' execution and delivery of this Agreement.

                  (f) The representations and warranties of the Credit Parties
         set forth in Section 1 of the Credit Agreement are true and correct as
         of the date hereof (except those that expressly relate to an earlier
         date) and all of the provisions of the Credit Documents, except as
         amended hereby, are in full force and effect.

                  (g) Subsequent to the execution and delivery of this Agreement
         and after giving effect hereto, no unwaived event has occurred and is
         continuing which constitutes a Default or an Event of Default.

         5. Liens. Each Credit Party affirms the liens and security interests,
if any, created and granted by it in the Credit Documents (including, but not
limited to, the Pledge Agreement) and agrees that this Agreement shall in no
manner adversely affect or impair such liens and security interests.

         6. Effect of Amendment. Except as expressly modified and amended in
this Agreement, all of the terms, provisions and conditions of the Credit
Documents shall remain unchanged and in full force and effect. The Credit
Documents and any and all other documents heretofore, now or hereafter executed
and delivered pursuant to the terms of the Credit Agreement are hereby amended
so that any reference to the Credit Agreement shall mean a reference to the
Credit Agreement as amended hereby.

                                       4
<PAGE>   5
         7. Expenses. The Company agrees to pay all reasonable costs and
expenses incurred in connection with the preparation, execution and delivery of
this Agreement, including the reasonable fees and expenses of the Agent's legal
counsel.

         8. Condition Precedent. The effectiveness of this Agreement is subject
to receipt by the Agent of executed counterparts (or other evidence of
execution, including facsimile signatures, satisfactory to the Agent) of this
Agreement, which collectively shall have been duly executed on behalf of each of
the Borrowers, the Subsidiary Guarantors and the Required Lenders (as evidenced
by consent letters).

         9. Counterparts. This Agreement may be executed in any number of
counterparts and by the parties hereto in separate counterparts, each of which
when so executed and delivered shall be deemed to be an original and all of
which taken together shall constitute one and the same instrument.

         10. Governing Law. This Agreement shall be governed by and construed in
accordance with the laws of the State of North Carolina.

         11. ENTIRETY. THIS AGREEMENT, THE CREDIT AGREEMENT AND THE OTHER CREDIT
DOCUMENTS EMBODY THE ENTIRE AGREEMENT BETWEEN THE PARTIES AND SUPERSEDE ALL
PRIOR AGREEMENTS AND UNDERSTANDINGS, IF ANY, RELATING TO THE SUBJECT MATTER
HEREOF. THESE CREDIT DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES
AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT
ORAL AGREEMENTS OF THE PARTIES.

                  [Remainder of Page Intentionally Left Blank]

                                       5
<PAGE>   6
         IN WITNESS WHEREOF, the parties hereto have caused this First Amendment
to Credit Agreement, to be duly executed and delivered by their proper and duly
authorized officers as of the day and year first above written.

THE COMPANY:                         HERCULES INCORPORATED,
                                     a Delaware corporation

                                     By:      /s/ G. MacKenzie
                                              ----------------------------------
                                     Name:    George MacKenzie
                                              ----------------------------------
                                     Title:   Executive Vice President and Chief
                                              ----------------------------------
                                              Executive Officer
                                              ----------------------------------

CANADIAN
BORROWER:                            BETZDEARBORN CANADA, INC.,
                                     an Ontario corporation

                                     By:      /s/ G. MacKenzie
                                              ----------------------------------
                                     Name:    George MacKenzie
                                              ----------------------------------
                                     Title:   Treasurer
                                              ----------------------------------
<PAGE>   7
OTHER SUBSIDIARY
GUARANTORS:
                                     HERCULES CREDIT, INC.,
                                     a Delaware corporation
                                     HERCULES FLAVOR, INC.,
                                     a Delaware corporation
                                     WSP, INC.,
                                     a Delaware corporation
                                     AQUALON COMPANY,
                                     a Delaware partnership
                                     HERCULES FINANCE COMPANY,
                                     a Delaware partnership
                                     FIBERVISIONS, L.L.C.,
                                     a Delaware limited liability company
                                     FIBERVISIONS INCORPORATED,
                                     a Delaware corporation
                                     FIBERVISIONS PRODUCTS, INC.,
                                     a Georgia corporation
                                     HERCULES INTERNATIONAL LIMITED,
                                     a Delaware corporation
                                     BETZDEARBORN, INC.,
                                     a Pennsylvania corporation
                                     BETZDEARBORN EUROPE, INC.,
                                     a Delaware corporation
                                     DRC, LTD.,
                                     a Delaware corporation
                                     BL TECHNOLOGIES, INC.,
                                     a Delaware corporation
                                     BLI HOLDINGS, INC.,
                                     a Delaware corporation

                                     By:      /s/ S. C. Shears
                                              ----------------------------------
                                     Name:    Stuart C. Shears
                                              ----------------------------------
                                     Title:   Vice President and Treasurer
                                              ----------------------------------
<PAGE>   8
ADMINISTRATIVE
AGENT:                                      BANK OF AMERICA, N. A.,
                                            for and on behalf of the Lenders
                                            in its capacity as Agent

                                            By:      /s/ David Noda
                                                   -----------------------------
                                            Name:    David Noda
                                                   -----------------------------
                                            Title:   Managing Director
                                                   -----------------------------

CANADIAN
ADMINISTRATIVE AGENT:                       BANK OF AMERICA CANADA,
                                            As Canadian Administrative Agent

                                            By:      /s/ R. J. Hall
                                                   -----------------------------
                                            Name:    Richard J. Hall
                                                   -----------------------------
                                            Title:   Vice President
                                                   -----------------------------

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