240 



PHOTO-MICROGRAPHY [CH. VIII 



desirable and for those above ioo it is necessary to use some form of ' ' vertical 

 illuminator," that is some arrangement by which the light is reflected down 

 through the objective upon the object, the objective acting as a condenser, 

 and from the object back through the objective and ocular to the eye of the 

 observer. This is accomplished in two ways: 



(i) By means of a small speculum-metal mirror in the tube of the micro- 

 scope. This is set at an angle of 45 degrees and the light thrown into the tube 

 upon it is reflected straight down through the objective upon the object. 

 The speculum metal being opaque cuts out a part of the light. Instead of a 

 metal mirror a circular disc of glass is now more frequently used. This allows 

 the major part of the light reflected from the object to pass up through the 

 objective, to reach the eye. 



(2) By means of a small glass 45 degree prism inserted into the side of 

 the objective or of a special adapter. The light is from the side of the micro- 

 scope, and is reflected by the prism straight down through the objective upon 

 the object as before.* See Figs. 185-186. 



§ 319. Light for the Vertical Illuminator. — For moderate 

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

 may 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 

 necessary 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 objec- 

 tives and different specimens. ■ The cone of light, especially with 

 the electric arc lamp, should be enclosed in a hollow metal or asbes- 

 tos cone, to avoid the glare in the eyes of the operator, and it may 



*The idea of the vertical illuminator apparently originated with Hamilton 

 L. Smith. He used the metal reflector. Beck substituted a cover-glass and 

 Powell and Lealand a disc of worked glass; i. e. glass that had been carefully 

 polished and leveled on the two sides. Carpenter-Dallinger, pp. 336-338. 



The use of the prism with the objective is due to Tolles (See Jour. Roy. 

 Micr. Soc, vol. iii, 1880, pp. 526, 574). 



In Zeiss' catalog the prism form is figured. In the catalog of Nachet both 

 the glass disc and the prism forms are figured. 



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 

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

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

 metallography. Indeed, without some such adequate method of illumination 

 the study of metals and alloys with high powers would be impossible. So suc- 

 cessful is it that oil immersion objectives may be used. (Carpenter-Dallinger, 

 PP- 335-338). 



