CH. VIII} PHOTO-MICROGRAPHY 239 



For both these devices uncovered objects are most successful or if the object 

 is covered it must be in optical contact with the cover-glass. Naturally good re- 

 flecting surfaces like the rulings on polished metal bars give most satisfactory 

 images, hence this method of illumination is especially adapted to micro-metal- 

 lography. Indeed, without some such adequate method of illumination the study 

 of metals and alloys with high powers would be impossible. So successful is it 

 that oil immersion objectives may be used. ( Carpenter- Dallinger, pp. 335-338). 

 383. Light for the Vertical Illuminator. For moderate 

 powers one may place the microscope in front of a window, or one ma}* 

 use a petroleum or gas lamp. For the higher powers acetylene or 

 preferably the electric arc light is used. In either case it may be neces- 

 sary to soften the light somewhat either by a color screen or by some 

 ground glass. The light should be concentrated upon the exposed end 

 of the prism or into the hole leading to the glass disc. Both the prism 

 and the disc should be adjustable for different objectives and different 

 specimens. The cone of light, especially with the electric arc light, 

 should be enclosed in a hollow metal or asbestos cone to avoid the glare 

 in the eyes of the operator, and it riiay be necessary to soften the light 

 with ground glass before attempting to focus and arrange the speci- 

 men. This ground glass would in most cases be removed before mak- 

 ing the exposure ( 379). 



With the electric light and for long exposure or observation a 

 water bath to absorb the heat rays will be necessary to avoid injuring 

 the lenses. (See also under projection in the next chapter). 



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 will serve for all but the finest focusing, and 

 thus avoid moving the tube for focusing enough to throw the lighting 

 out of adjustment. It might be advantageous to have a fine adjust- 

 ment on the stage also. 



\ 384. Mounting of Objects. For observation only and with low powers, 

 the 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. 



385. 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 alumnina, etc). The aim is to get rid of the scratches so 

 that the surface will be smooth and free from lines. 



\ 386. Etching. After the surface is polished it should be etched with some 

 substance. This etching material will corrode the less resistant material, the 

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



