61 



side toward the object ; if an ordinary hand lamp is 

 used, it will be necessary to elevate this to about the 

 height of the eye-piece, and if it is to be used often 

 in this position, a special support should be made for 

 this purpose. 



Low power objectives are usually used on opaque 

 objects, but sometimes a higher power is desired. 

 Unless one is constructed with a view to opaque illumi- 

 nation its working distance is usually so short that it 

 will prevent the light from striking the object. A J 

 or i objective, of 75 degrees, has sufficient working- 

 distance, and its mounting are made conical in the 

 front, so that it will allow it. 



Dark Ground Illumination. This method is 

 not in general use, probably because it requires a 

 special accessory, although it yields beautiful effects. 

 It is accomplished by means of a paraboloid (Fig. 17) 



which is attached to the 

 sub-stage. As will be no- 

 ticed in the illustration, the 

 lower surface of the para- 

 poloid is plane, and the 

 light passes through this 

 without undergoing any 

 change. When it reaches 

 the polished parabolic sur- 

 face it is reflected to one 

 point, according to the sim- 

 ple optical law that the 

 Ffg. 17. angle of reflection is equal 



