172 



THE MICROSCOPE. 



matic condenser of a very long focus and large aperture, 

 mounted in such a way as to enable its axis to be inclined 

 to that of the microscope through a wide angular range ; 

 a condenser of this construction, he says, is also suitable 

 for all ordinary purposes. 1 A combination of the reflect- 

 ing and refracting powers of a prism is much preferred by 

 some ; such, for instance, as that known as Amici's prism, 

 which causes the rays to be reflected by a plane surface, 

 and made to converge by a lenticular one, so that the 

 prism answers the double purpose of mirror and condenser 

 at the same time. "Abraham's Achromatic Lenticular 

 Prism" is an excellent substitute for the achromatic con- 

 denser, and answers the double purpose of mirror and con 

 denser. This prism, represented half-size in fig. 101, is made 



Fig. 101. — Abraham's Achromatic Lenticular Prism. Half-size. 



up of two kinds of glass, set in a frame of brass ; the part 

 employed as the reflector (a) is of flint glass, hollowed out at 

 its upper surface, and into this is accurately fitted a double 

 convex lens of crown glass (b), so contrived as to have a 

 focus of about four inches. By the tube (g), the prism can 

 be applied to the ordinary support of the mirror ; and by 

 means of the flat semicircle (d), and a joint in the connect- 

 ing piece (f), it can be turned in every possible direction, 

 the semicircle sliding through a spring clip at e. By this 



(1) Quart. Journal Micros. Science, vol. iii. p. 87. 



