The Miceoscope. 173 



For low powers, where a geographical picture of a tissue is wanted, 

 it leaves nothing to be desired, giving a perfect black and white 

 negative. 



Of the different illuminating methods for photo-micrography 

 sunlight is preferable; where it is not possible for the operator to 

 obtain a heliostadt, a good sized mirror, swung on a double axis 

 and regulated by hand will fill all the requirements; as the exposure 

 by sunlight is of so short a duration, it is quite easy to keep the 

 light centered. In using sunlight as the illuminant, the light should 

 be passed through a cell of saturated solution of alum, in order to 

 extract the heat rays ; otherwise, unless great care be taken, there is 

 danger of melting the cement of the substage condenser or objective. 



The photographic process employed, will determine the color of 

 the screen to be placed between the source of light and the micro- 

 scope, to render the light monochromatic. For the wet-plate 

 process, a cell of ammonia sulphate of copper is used. In dry-plate 

 work, for the orthochromatic process, instead of the ammonia sul- 

 phate of copper cell, one of a saturated aqueous solution of picric 

 acid is used; this gives a yellowish orange light, which, with the 

 orthochromatic dry places, it is possible to get successful pictures of 

 tissues stained by any dye, although the reds and blues give the best 

 results. 



After arranging light, color-screen, microscope etc., in their 

 proper order, before successful work can be accomplished it will be 

 found necessary to have a proper focusing- screen to the camera. 

 For high powers and fine focusing, the finest ground glass will be too 

 coarse, the best substitute will be a gelatine plate. To prepare this 

 focusing-screen, place a gelatine plate in a holder, and underexpose, 

 then develop, fix and wash thoroughly, as for an ordinary exposure; 

 the plate thus developed will show a slight milkiness, and yet be 

 perfectly transparent; place this plate in the camera with the film 

 surface toward the microscope. This will give a focusing surface, 

 on which the finest cell can be distinctly seen, and on which a focus- 

 ing glass may be used to advantage. 



For cheapness and at the same time excellent results, the wet 

 process must certainly rank foremost, though the knowledge neces- 

 sary will always be a bar to its general use by amateurs. 



Of the different diy plates in use, the ordinary dry plate will 

 be found a snare and a delusion for photo-micrography, except, 

 perhaps, for bacteria work. The orthochromatic and the isochro- 

 matic are the only dry plates which will insure results equal to or 



