MEDIUM-POWER PHOTO-MICROGRAPHY 



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apparatus we have previously mentioned — one for low-power work and the other for 

 " tube " work — without any loss of time, as the same block fits both, the rails being 

 made the same width apart. Besides this, removal of the entire camera allows the 

 photographer the opportunity to examine his specimen sufficiently for all purposes 

 provided he has previously found the " best place." When we speak later upon 

 " taking the photograph " we shall refer to this again. Doing away with the rotating- 

 board is an economy, and its loss we have but rarely felt. 



With respect to the ground glass screen-holder of the camera, we have had it 

 made in such a fashion that the glass itself can be slipped out by the release of two 

 springs, and others, whether plain, smoked, or more finely ground, easily substituted. 

 Turning these springs on one side, the glass falls out ; turning them back again, 

 another can be fixed. With respect to the kinds of ground glass employed, we use 

 three varieties : — The ordinary coarse-grained one, another of finer texture, and a 

 third made specially for us by Ross, Ltd., which is so fine as to hardly be worthy of 

 the name of ground glass at all. When commencing to take a photograph either the 

 first or second is used, and after illuminating the field equally, in a manner to which 

 we shall presently refer, one proceeds to focus ; if that can be done on the coarse or 

 medium glass we are always more satisfied ; but if with extremely difficult objects the 

 ability to do so is not certain, we change to the finest type or even use a piece of 

 plain glass. The objection to using plain glass or the finest of the ground glasses, in 

 the first instance, is that the general appearance of the field is entirely lost : you see 

 nothing with the naked eye but light streaming from the eyepiece, and, therefore, 

 any inequality of the illumination is sure to escape notice. But for the more accurate 

 focussing that is required with the secondary markings of diatoms or the flagella of 

 bacteria we resort to it accompanied by the use of a focussing-glass, which is shown in 

 the diagram near the foot of the camera. Much diffisrence of opinion exists as to 

 what kind of magnifying glasses it is better to use. 



Dr. Bousfield, an eminent photo-micrographer, advocates the use of a lens with as 

 low a power as possible ; others prefer a spectacle lens mounted in a frame much 

 like an ordinary pair of spectacles, having about an inch focus ; whereas not a few 

 prefer the ordinary photographic hand-magnifier, which has already been referred to, 

 providing it is achromatic. Personally, we have a predilection for this type, and use 

 one made by Dallmeyer, which he terms his " Low-power focussing hand-magnifier." 

 Before using a hand-magnifier, however, it should be carefully focussed on the same 

 ground glass upon which it is to he employed. This is not quite so easy a matter as 

 one would think, for it requires a little practice to know when the glass grinding 



