I can contribue two techniques I know have worked well.

(1)  I built a Peltier - heated/cooled device with a think aluminum plate
that extended into the area of the objective, that had a calibrated
miniature thermopile inserted into a Sylgard embedment.  I could directly
read off the temp while doing experiments.  I could hold the temp to +/-
0.1 deg C manually; worked well for what I did.  Cost was mainly the
Peltier itself with a water jacket and a good power supply, total of
about $400 about 8 years ago.  I can describe in greater detail for you if
you like.

(2) A colleague at my previous institution merely built a housing around
the entire stage and condenser setup, used simple resistance heaters and
dryer ducting and heated the entire setup to 37 deg C routinely.  It was
very effective and I think cost hardly anything.
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On Tue, 23 Jun 1998, Kees Jalink wrote:

> With respect to the objective heater, we have been using a very cheap,
> homebuild alternative that works very well to warm the objectives to 37
> degrees. It consists of a heater filament (or power resistor or the
> like) and a small fan (diameter 5 cm or so) that gently blows the heated
> air over the objective and the revolver. A temp sensor with feedback
> loop keeps the air temp constant. Works great, allows for objective
> changes, does not cause any fibrations, and costs less than 100$
>
> more details on request.
> greetings. Kees Jalink
>
> --
>
> Kees Jalink Ph.D.
> The Netherlands Cancer Institute, dept. of Cell Biology H1
> Plesmanlaan 121 1066CX Amsterdam, the Netherlands
> 020-5121933 (tel)   / 020-5121944 (fax)
> [log in to unmask] (email) / 0297-320248 (tel at home)
>