PRACTICAL PHOTO-MICROGRAPHY. 73 



a bulPs-eye may be placed between light and condenser, at a 

 possible sacrifice. With high power objectives the image of 

 the flame, focused as before, is usually large enough to illu- 

 minate the whole field without any bull's eye. With the low- 

 est powers the condenser is usually and preferably omitted. 



NOTE. In very many cases it will be found convenient and 

 advantageous to use an objective as substage condenser. The 

 objective so used may have an aperture equal to or less than 

 that of the objective used for the image projection. Those 

 who have good objectives, but only poor condensers, may do 

 well to adopt this system for all work. The writer, for pur- 

 pose of experiment, had a mount made fitting his substage and 

 taking his objectives ; the addition of a small iris diaphragm 

 made the apparatus complete. The results were so satisfactory 

 that we would not hesitate to adopt this system entirely, were 

 we not already in possession of Powell and Lealand's fine apo- 

 chromatic, and Zeiss' achromatic condensers; the former of 

 !N". A. 1-4, the latter of N. A. 1, and capable of being used for 

 quite low angles. For example, using in our. substage as con- 

 denser an apochromatic o. g. of 16 mm. "N. A. .30, we resolved 

 with a 2 mm. o. g. P. angulatum,into white areas or black dots, 

 with ease and at will. For ordinary photography of bacteria 

 and the like we find this arrangement, or a similar one, not at 

 all inferior to the use of expensive achromatic condensers. 



