192 PHOTOMICROGRAPHY 



useful in giving an appearance of relief to the finished 

 photograph. If a powerful illuminant is used, a water- 

 trough should be placed between it and the condenser, 

 or concentration of the heat rays may ruin the slide. 

 When the objects to be photographed are solid and fairly 

 large, it will be found necessary to place a stop in or above 

 the objective to increase the depth of focus. A set of 

 cylindrical metal stops having apertures of J", iV", and 

 ^V' in diameter may be made to fit in the objective 

 mount between the component glasses, or the Davis 

 diaphragm can be used for the purpose, and is frequently 

 more convenient, though not quite so satisfactory. 



On account of the relatively small amount of light 

 reflected from the object, a very long exposure will be 

 required, particularly if the objective is stopped down to 

 any great extent, and for this reason an intense illumina- 

 tion is necessary for this kind of photomicrographic 

 work. The presence of a cover-glass on the preparation 

 may give rise to reflections, and, whenever possible, 

 should be removed before photographing. 



There are other means of illuminating opaque objects, 

 namely, by the use of a parabolic side reflector or a 

 Lieberkuhn ; but, as a rule, the results obtained by 

 these are in no way superior to those that can be pro- 

 duced by a careful adjustment of the bull's-eye condenser, 

 and the latter should form part of every microscopist's 

 outfit. One form of the parabolic reflector is shown at 

 Fig. 64 among the accessories supplied with the Rosenhain 

 microscope. It is a little silvered mirror carried in the 

 optic axis of the microscope by a long dovetailed slide 

 which fits into a groove on the body. Light is thrown on 

 to the reflector by means of a condenser, and focussed on 

 to the object by sliding the reflector up or down. The latter 

 nearly touches the object or slide when in position. 



Stereoscopic Photomicrography. The best representa- 

 tion of the forms of small solid objects is obtained by 



