Oblique Light. Even though both the condenser 

 and iris are' centered, central or axial light is not ob- 

 tained, unless the rays of light, or the axis of the cone 

 of light, from the mirror enter the condenser parallel 

 with its axis. This fact is often forgotten as was pointed 

 out before. 



Beside the turning of the mirror oblique light can be 

 obtained in a greater degree by decentering the lower 

 iris and in the best microscopes the obliquity can be 

 obtained from any azimuth'by revolving the diaphragm 

 mounting. 



With the simple diaphragm mounting with a ring 

 beneath the diaphragm for blue glass oblique light can 

 be obtained by slipping a card between the ring and 

 diaphragm in such a manner as to let the light into one 

 side of the condenser only. 



ILLUMINA TIONOPA QUE 



There are some objects which cannot be made trans- 

 parent and must be examined by reflected light. When 

 low powers are used and the mirror brought above the 

 stage the concave mirror is sometimes sufficient. The 

 bull's eye condenser gives better results. In using it 

 some care must be exercised to light the object and at 

 the same time cast as few shadows as possible. 



Where high powers with consequent short working 

 distances are necessary, light must be thrown down 

 through the objective and reflected back again to the 

 eye by a prism placed in the objective mount or above 

 it. Artificial light is necessary with such an arrangement. 



FOCUSING THE CONDENSER 



Nearly every substage is provided with a means for 



focusing the condenser. The condenser does its best 



work only when the rays of light passing through it 



are focused sharply on the object. If there are any 



21 



