Tim,
If you do not already have good reference material on this microscope you might want to check these out:
http://www.reichertms.com/Manuals/Micro ... Manual.pdf
http://www.reichertms.com/ref_man.php
http://www.xmission.com/~psneeley/Perso ... oscope.htm
Unless the objective is marked with the "infinity" symbol I don't think you can distinguish between "finite" and "infinity" by just looking at them. Of course American Optical (bought by Reichert, bought by Leica) is widely known for "infinity" optics, but all the ones I've seen have been clearly marked.
Fortunately there is much good information still available for these microscopes. I'm pretty sure all the AO infinity objectives would work, and they are plentiful, rather inexpensive, and generally excellent in quality. I don't know much about the objectives marketed by Reichart and then Leica after they absorbed AO, but a little time spent looking over the above sites should be helpful. "Product Numbers" and these old catalogs and price lists are sometimes a big help in determining what you are looking at. Be sure you are using "infinity" objectives on that stand.
A 4X is a good power on a compound scope for small insects. A 10x will be getting you into small details of small insects. Generally there is a relay optic in the trinocular head that will multiply the objectives "power" by a factor somewhere between 1.5 and 2.5X before it reaches the camera, so you will have taken a pretty big jump from 1:1. (The range between about 1:1 and 6:1 is not always that easy to establish on a compound microscope. Good objectives below 4X are not all that common, and working distance is always an issue).
I have mounted AO infinity objectives on a bellows just out of curiosity. You can get an image but I have not tested the quality. They were designed to be used with a "tube lens" to form a real image, so mounted on a bellows (without the tube lens) you are using them at a "lens to subject" distances not intended in order to form the image at the film/sensor plane. (This may not be a really big issue with low power, low NA objectives). Microscope objectives on bellows is an interesting concept. Rik has gotten some super results. My best results have been with certain Nikon "CF" objectives. Most "finite" objectives were designed so that some chromatic aberrations were to be corrected by the microscope eyepieces. The Nikon CF series were fully chromatically corrected in the objective. Some "infinity" microscopes do some chromatic aberration correction in the tube lens. So it's all something of a gamble... you just try it and see how it looks. (Also don't forget that reversed quality enlarging lenses also do nicely on bellows).
Good luck and let us know how things progress.
Charlie
.