ACCESSOEY INSTRUMENTS. 



119 



the best advantage, in consequence of all the rays being re- 

 flected from the same surface, which is not the case with a 

 silvered glass mirror. 



Achromatic Prism and Condenser. — This very important in- 

 strument, answering the purpose both of mirror and achromatic 

 condenser, was presented to the author by Mr. Abrahams, 

 optician, of Liverpool, and is shewn of the natural size in fig. 

 64. The prism is made up of two kinds of glass, set in a 



Fig. 64. 



frame of brass ; the part employed as the reflector. A, is of flint 

 glass, hollowed out at its upper surface, and into this is accu- 

 rately fitted a double convex lens of crown glass, B, so contrived 

 as to have a focus of about four inches. By the tube, Gr, 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 con- 

 necting piece, F, it can be turned in every possible direction, 

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

 instnmient achromatic condensed light may be thrown 

 upon any object on the stage. The prism has the usual 

 swinging motion, accomplished by the frame turning on two 

 screws, one of which is seen at C, at the end of the semi- 

 circle. 



Oblique Prism. — This instrument, invented by M. Nachet, 

 of Paris, is represented in section by fig. 65 ; it consists of a 

 prism of glass, having both its surfaces, a b and c d, convex, 

 by which means the rays of light, I, reflected from the mirror. 



