758 



Transactions of the Society. 



tionally small front leus, but it will serve to show the convenient 

 ajoplication of the prism to objectives of this class. 



The rays a and h in Fig. 74, as traced by Professor Keith, show 

 an angle of 110°, or 55° on each side of the axis. Eays, whether 



Fig. 74. 



parallel or divergent, entering the prism at c, would take the same 

 general direction as the rays a and 6, but their focal distance would 

 be about thrice that of the entire objective. If the seat of the 

 prism, as shown in Fig. 73, is in a plane at right angles to the 

 optical axis, then the direction of the illuminating ray can be con- 

 siderably controlled by raising or lowering the outer end of the 

 prism by means of the screw at /, and the reacting spiral spring 

 above." 



