Boyal Microscopical Society. 149 



mating lens is 1£ inch iu diameter with a focal length of 4 inches. 

 The prisms should be of the same width as the collimating lens ; 

 and, as Dr. Eobinson and others have suggested, they might be 

 made up of any of the forms of compound prisms which were con- 

 sidered most suitable to the object for which this spectroscope 

 was chiefly selected. The magnifying power required is best 

 obtained by changing the objective. 



