132 
ELEMENTARY CHEMICAL MICROSCOPY 
the comparing prism p beneath the slit or out to one side. T 
attached to a right and left threaded spindle serves to widen or 
narrow the slit 5. Attached to the upper part of K is the re¬ 
mainder of the eyepiece with its eye lens e vertically movable by 
rack and pinion through the milled head F. Fitting above e is 
Fig. 66 . Microspectroscope. (W. & H. Seibert.) 
a tube A carrying an Amici prism R consisting of three prisms of 
crown glass {fiD = i-534) alternating with two prisms of flin t 
glass {fiD = 1.587)- 
Since the total deviation of a ray of light entering a series of 
prisms is equivalent to the sum of the deviations which would be 
imparted to it by each unit in turn, it follows from the alternate 
