240 PHOTO-MICROGRAPHY [CH. VIlI 



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 indiffer- 

 ent liquid. See numerous articles in the Metallographist for methods and micro- 

 graphs. 



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

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

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

 then last for several months untarnished. 



I 387. Mounting the 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 beeswax. (Behrens). Very elaborate arrangements of the stage 

 have also been devised (Reichert). A simple and effective device is shown in Fig. 

 193 in which the specimen is held against the under side of the plane face of the 

 stage attachment. Rubber bands answer well to support the metal, and only one 

 side need be flat. 



§ 388. Photographing Opaque Objects. — The general directions given in § 347 

 should be followed with the necessary modifications. The time of exposure is 

 usually considerably greater with opaque objects than with transparent ones. 

 Very few such objects can be photgraphed in less than 30 seconds, even with day- 

 light For metallic surfaces and magnifications of 100, 150, 250 to 500, with the 

 electric arc light as illuminant the time required for favorable objects is I, 2, 

 4 and 7 seconds ; with the Wellsbacb lamp the time is 5, 10, 30 and 60 minutes 

 (Sauveur ). 



Fig. 194. Rack for drying negatives 

 (Rochester Opt. Co). 



Fig. 194. 



References to Ch. VIII. 



See the works and journals dealing with photography. 



For Photo-Micrography see Pringle, Bousfield, Neuhauss, Sternberg, Francotte 

 and the special catalogs on photo-micrography and projection issued by the great 

 opticians. The Journal of the Royal Microscopical Society and of the Quekett 

 Micr. Club; Zeit. wiss. Mikroskopie ; the Trans Amer. Micr. Soc; the Amer. 

 Monthly Micr. Journal ; the Journal of Applied Microscopy. 



For the photography of metallic surfaces, see the various journals of engineer- 

 ing and metallurgy, but especially Sauveur's journal, the Metallographist, begun 

 in 1898. 



