Exhibit 12 b

                                                                   March 1, 2000

Thermo Instrument Systems Inc.
81 Wyman Street
Waltham, MA 02454
Attn:  Earl R. Lewis

Dear Earl:

     Reference is made to Thermo Instrument's proposed acquisitions of the
minority interest investment in each of its majority owned subsidiaries,
ThermoQuest Corporation, Thermo Optek Corporation, Thermo BioAnalysis
Corporation, Metrika Systems Corporation and ONIX Systems Inc. In connection
with those transactions, Thermo Electron Corporation intends to loan Thermo
Instrument up to $400,000,000 for the acquisition of such interests in each
of ThermoQuest Corporation, Thermo Optek Corporation, Thermo BioAnalysis
Corporation, Metrika Systems Corporation and ONIX Systems Inc. This loan will
be available at an interest rate equal to the 30-day Dealer Commercial Paper
Rate plus 150 basis points, set at the beginning of each month, provided such
rate shall be reduced to the Dealer Commercial Paper Rate plus 50 basis
points to the extent of any funds invested by Thermo Instrument's majority
owned subsidiaries in Thermo Electron's cash management arrangement. Interest
will be due and payable on a monthly basis and the loan will be payable in
full on September 1, 2000. If these terms are acceptable to you, please sign
the letter as indicated below to evidence your acceptance of the terms.

                                    Sincerely,
                                    THERMO ELECTRON CORPORATION

                                    /s/ Theo Melas-Kyriazi
                                    ----------------------
                                    Theo Melas-Kyriazi
                                    Vice President and Chief Financial Officer

ACCEPTED AND AGREED TO:

THERMO INSTRUMENT SYSTEMS INC.

/s/ Earl R. Lewis
- ---------------------------------
Earl R. Lewis
President and CEO