CH. VIII] PHOTO-MICROGRAPHY 



225 



phragms so that the aperture of the condenser may be varied depending upon 

 the aperture of the objective. For a long time objectives have been used as 

 achromatic condensers, and they are very satisfactory, although less conven- 

 ient than a special condenser whose aperture is great enongh for the highest 

 powers and capable of being reduced by means of diaphragms to the capacity 

 of the lower objectives. It should also be capable of accurate centering 

 (192)- 



I 303. Objects Suitable for Photo-micrographs. — While almost any large 

 object may be photographed well with the ordinary camera and photographic 

 objective, only a small part of the objects mounted for microscopic study can 

 be photo-micrographed satisfactorily. Many objects that can be clearly seen 

 by constant focusing with the fine adjustment, appear almost without detail 

 on the screen of the photo-micrographic camera and in the photo-micrograph. 



Fig. 175. Engraving glass to serve as a con- 

 denser and for a dissecting lens. (Bausch & Lomb 

 Opt. Co.) 



If one examines a series of photo-micrographs the chances are that the 

 greater number will be of diatoms, plant sections or preparations of insects. 

 That is, they are of objects having sharp details and definite outlines, so that 

 contrast and definiteness may be readily obtained (§ 107, 118, 157). Stained 

 microbes also furnish favorable objects when mounted as cover-glass prepara- 

 tions, but these give color images (§ 107, 119) and require a color screen 



(?2 9 l). 



For success with preparations of animal tissue they must approximate as 

 nearly as possible to the conditions more easily obtained with vegetable prep- 

 arations. That is, they must be made so thin and be so prepared that the cell 

 outlines have something of the definiteness of vegetable tissue. It is useless 

 to expect to get a clear photograph of a section in which the details are seen 

 with difficulty when studying it under the microscope in the ordinary way. 



Many sections which are unsatisfactory as wholes, may nevertheless have 

 parts in which the structural details show with satisfactory clearness. In such 

 a case the part of the section showing details satisfactorily should be sur- 

 rounded by a delicate ring by means of a marker (see Figs. 70, 72). If one's 

 preparations have been carefully studied and the special points in them thus 

 indicated, they will be found far more valuable both for ordinary demonstra- 

 tion and for photography. The amount of time saved by marking one's speci- 

 mens can hardly be overestimated. The most satisfactory material for 

 making the rings is shellac colored with lampblack. 



Ten years ago many histologic preparations could not be satisfactorily 

 photographed. Now with improved section cutters, better staining and 

 mounting methods, and with the color screens ($ 291) and isochromatic plates 



