70 Royal Society : — 



a refracting angle of 6° for one of the outer plates of each prism— 

 the base of this crown-glass prism being brought to correspond with 

 the apex of the fluid prism, thus : — 



Crown-glass prism. 



aiiiiiiiji 



By this means the angle of minimum deviation of the prisms is so 

 much decreased, that eleven of them thus constructed can be used 

 in a circle instead of eight. An increase of dispersive power, due to 

 refracting angles of 150° of the bisulphide of carbon, is thus gained, 

 minus only the small amount of dispersion counteracted owing to 

 the dispersive power of the crown-glass prisms being employed in 

 the contrary direction. 



From the well-known low dispersive power of this medium, however, 

 this loss is inconsiderable, amounting to scarcely more than a fifteenth 

 of the power gained. Owing to the minimum angle of deviation being 

 lowered, the further advantage is also secured of a larger field of view 

 being presented to the telescope by the first and last prism of the train. 



Each prism, in addition to the 

 light metal frame referred to, has 

 a separate stand, furnished with 

 screws for adjusting the prisms, 

 and securing them at the angle 

 of minimum deviation for any 

 particular ray. The prism stands 

 within a stirrup furnished with a 

 welled head. By this arrangement 

 the prisms can be removed and 

 replaced without touching their 

 sides — a matter of some import- 

 ance, as all fluid prisms show dif- 

 ferent results with every change 

 of temperature. 



For the sake of simplicity, the 



* Direction of ray as it would pass through two pair of parallel sides 

 t Direction of ray as altered by interposing the crown-glass prism. 



