PHYSIOLOGY OF THE INVERTEBRATA. 



173 



of comparison, which is brought to it by a superimposed 

 reflecting prism and lateral tube d with collimator lens, 

 diaphragm carrier m, and mirror S, or incandescent lamp. 



In the upper opening of the box A is placed either an 

 eyepiece in a sliding jacket, which is accurately adjusted to 

 the slit ; or, instead of this (after proper adjustment of the 



Fig. 33. — The Engelmann Microspectrometer. 



image of the specimen in the objective slit) the spectroscopic 

 apparatus a'A'BO, which is fixed in the proper azimuth by 

 an arresting mechanism. This apparatus consists of the box 

 A' which on one side (the upper end of a') contains a col- 

 limator lens I, to render parallel the cone of rays proceeding 

 from the objective before they fall on a Eutherford prism P 

 of great dispersion. By the lens I' on the other side (at the 



