238 PHOTO-MICROGRAPHY [CH. VIII 



Such a microscope has the following general characters : The stage is mova- 

 ble up and down with rack and pinion, it is rotary and more or less mechanical by 

 means of centering screws. With some at least the stage may be removed 

 entirely. No substage condenser is present, and a mirror is only present for occa- 

 sional transparent objects. A revolving nose-piece is not so good as the objective 

 changers. 



\ 382. Illumination of Opaque Objects. — (A) for 25 to ico diameters. The 

 directions of Mr. Walmsley are excellent (Trans. Amer. Micr. Soc, 1898, p. 191). 

 "Altogether the best light for the purpose is diffused daylight. Proper lighting is 

 more easily obtained with a vertical camera. An even illumination avoiding deep 

 shadows is preferable in most cases and is more easily attained with the object in 

 a horizontal position. For many objects it is better not to use a bull's eye or any 

 form of condenser but for others the condenser may be needed, but when the con- 

 denser is used one must avoid too much glare. The now little used parabolic re- 

 flector and Lieberkiihn serve well in many cases, but he adds "the majority yield 

 better results under the most simple forms of illumination," i. e., with the dif- 

 fused light from the window. This has been the experience of the writer also. 



In case diffused daylight is employed the camera should be near a good sized 

 window, and the object should be somewhat below the window ledge so that the 

 illumination is partly from above and from the side. (This is easily attained with 

 the small table and vertical camera shown in Figs. 175, 184). The vertical illum- 

 inator is advantageous for these powers also. See (B. ). 



(B) For 100 to 500 diameters, — For the magnifications above 50 it is desirable 

 and for those above 100 it is necessary to use some form of "vertical illuminator," 

 that is some arrangement by which the light is reflected down through the objec- 

 tive 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 accom- 

 plished in two ways : 



( 1 ) 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, 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 microscope, and 

 is reflected by the prism straight down through the objective upon the object as 

 before.* 



*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 Zealand 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. 



