PRODUCING STEREO-PHOTO-MICROGRAPHS. 289 



to remember, when photographing opaque objects, that, to secure 

 even illumination of the two halves, the illuminant must be on 

 the same level as the centre of the objective. For the lighting 

 of large transparent objects, when using the 4| and G-in. objec- 

 tives, I have had a cell which carries a 4|, G, or 8 -in. focu- 

 uncorrected condenser fitted to the large aperture under the main 

 stage of the microscope, and brought as near as possible to the 

 object. For the smaller objects I use an achromatic condenser 

 of 1*0 N.A. This can be altered in power by removing the 

 top combination, or, if necessary, using only the lower of the 

 three lenses. As illuminant I use acetylene (the finest light for 

 all ordinary work) from a special burner I had made, limelight, 

 and sometimes sunlight (parallel rays) through a heat-absorbing 

 medium. On the platform carrying the apparatus I have marked 

 a scale from to 49 in., with the zero at the level of the micro- 

 scopic stage. This, with the aid of tables for the various lenses 

 used, enables me to work to definite magnitudes, and also to 

 calculate approximately the focus of all objectives that can be 

 used without an eye-piece. 



I might remark in passing that low-power micro -objectives 

 are almost invariably wrongly named, a nominal 3 -in. being 

 only 2 "3 in., a so-called 2 -in. only 1*5 in., and so on. I do not 

 think I need say anything about exposure, as that depends upon 

 the colour or brightness of the object, illumination, and magnifica- 

 tion. I use ordinary developers, such as would be suitable for 

 landscapes, using those which give hard results to accentuate 

 feeble contrasts. Chromatic plates, with or without a yellow 

 screen, are used, and a malachite-green light-filter when mono- 

 chromatic light is desirable. A good deal of my apparatus is 

 makeshift, but is effectual, and my optical outfit is of the best, 

 both in low and high powers, condensers, etc. I might mention 

 that (as the major conjucate focus is being used) all non-sym- 

 metrical photographic lenses, such as " Unars," " Stigmatics," 

 and portrait combinations, should be reversed on the microscope 

 (the front combination being presented to the plate) to enable 

 them to perform to the best advantage. 



Before concluding, I would like to draw attention to a curious 

 (and to me interesting) optical defect noticeable in one of the 

 prints. In a photograph of a slide of polyzoa (Idmonea radians) 

 I noticed the distorted ima^e of a small filament, which was on 



