USEFUL MICROSCOPE ACCESSORIES 



107 



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 s. 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. 54. Microspectroscope. (W. & H. Seibert.) 



a tube A carrying an Amici prism R consisting of three prisms of 

 crown glass (UD = 1.534) alternating with two prisms of flint 

 glass (n D = 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 



