CH. 1 7/7] PHOTO-MICROGRAPHY ■ 241 



be uecessary to soften the light with ground glass before attempt- 

 ing to focus and arrange the specimen. This ground glass would 

 in most cases be removed before making the exposure (§ 314.) 



With the electric light and for long exposure or observation, a 

 water bath to absorb the heat rays is necessary to avoid injuring the 

 lenses. 



As it is somewhat difficult to adjust the light in a way to give 

 the best effect, one can see the advantage of the adjustment for 

 raising and lowering the stage. This serves for all but the finest 

 focusing, and thus avoids moving the focusing tube enough to 

 throw the lighting out of adjustment. It might be advantageous to 

 have a fine adjustment on the stage also. 



I 320. Mounting of Objects. — For observation only and with low powers, 

 objects may be mounted either in a liquid or dry as seems best. There should 

 be a black background for most objects, then light will reach the eye only 

 from the object. A light background is sometimes desirable, especially where 

 one cares only for outlines. 



I 321. Preparation of Metallic Surfaces. — In the first place a flat face is 

 obtained by grinding or filing, and then this is polished. For polishing, finer' 

 and finer emery or other polishing powders are used, (rouge or diamantine, or 

 specially prepared alumina, etc). The aim is to get rid of scratches so that 

 the surface is smooth and free from lines. 



I 322. Etching. After the surface is polished it should be etched with 

 some substance. This etching material corrodes the less resistant material, the 

 edges of crystals, etc., so that the structure appears clearly. For etching,, 

 tincture of iodine, nitric acid in various degrees of strength, hydrochloric acid, 

 etc., are used or one may use electricity, the metal being immersed in an indif- 

 ferent liquid. See numerous articles in the Metallographist for methods and 

 micrographs. 



After etching, the surface should be washed well with water to remove the 

 etcher. Le Chatelier recommends that the etched surface when dry be covered 

 with a very thin coating of collodion to avoid tarnishing. The preparation 

 will then last for several months untarnished. 



I 323. Mounting Preparations of Metal. — In order to get a satisfactory 

 image the flat, polished and etched face should be at right angles to the optic 

 axis. For preliminary observation one can approximate this by mounting the 

 specimen on a piece of bees-wax. (Behrens). Very elaborate arrangements of 

 the stage have also been devised (Reichert). 



\ 324. Photographing Opaque Objects.— -The general directions given 

 in I 282 should be followed with the necessary modifications. The time of 

 exposure is usually considerably greater with opaque objects than with trans- 

 parent ones. Very few such objects can be photographed in less than 30 



