CONSTRUCTION OF PRISMS. 



463 



angles with the refracting surfaces, the prism produces 

 the 4 least deviation ; ' in all other cases the deviation 

 becomes somewhat greater. 



The common kinds of glass prisms, which may be purchased, 

 have mostly a section which is an equilateral triangle. Their re- 



Pie. 263 ( an. proj., real size). 



fracting angle is therefore 60. They produce generally very irre- 

 gular refractions, because the glass is not of equal density through- 

 out, and on account of their large refracting angle they give rise to 

 the phenomenon of dispersion of light, which will be discussed in the 

 next article. Such prisms are therefore not very well adapted for 

 our present experiments, for which prisms of water, with moderately 

 large refracting angles, are more suitable. Such prisms may be 

 prepared pretty easily in various ways. 



Fig. 263 A represents a small wedge-shaped board of hard wood, 

 which should be cut by a joiner ; it is about 5 cm long and wide ; one 

 end is very thin, and the other from 9 to 18 mm thick. A circular hole 

 is bored through the middle with a centre-bit, afterwards widened 

 with a keyhole-saw or a sharp knife, and finally rounded off with a 

 half-round file. The space within the hole is to be filled with 

 water, and in order to clamp the water prism conveniently, in the 

 retort-stand, a wooden handle, s, is inserted into the side of the 



