106 Modern Microscopy 



and the other with a disc of cover-glass. Metallurgists 

 usually have the two forms, finding one more serviceable 

 than the other under certain conditions and with different 

 specimens. They both have their distinct value. Gener- 

 ally speaking, for this class of work the prism pattern is 

 the better for low-power objectives, and the disc form for 



the high powers. The latter is 

 illustrated in Fig. 39, and may 

 be either built in the body of 

 the microscope or screwed to 



the nosepiece of the microscope 

 Fio. 39. — Vertical oe Disc . ,, , ■ ,• ,i 



Illuminator. above the objective; the one 



illustrated is arranged for the 



latter. The disc of cover-glass attached to a little clip 



having a milled head a is placed through a slot in the 



fitting b, and set at an angle of 45° to the optic axis. 



Light is received on this disc of glass through the small 



opening in the body of the fitting b, and it is totally 



reflected through the object-glass on the object, the 



objective acting as its own achromatic condenser. In 



order that the light may be focused on the object, the 



lamp-wick from which the light is being obtained must be 



the same distance from the reflector as the latter is from 



the diaphragm of the eyepiece, if a positive eyepiece is 



being used, or to the eye lens, if a Huyghenian or negative 



eyepiece is employed. 



