462 REFRACTION THROUGH PRISMS. 



aperture of about 2 cm in diameter; about l cm from the 

 sheet of card-board the screen used for the experiments 

 in Art. 38 is placed. Close behind the aperture is the 

 prism, edge upwards, clamped in the fork of the retort- 

 stand. The beam of light which passes through the 

 aperture previous to the interposition of the prism 

 produces a bright circular spot at a upon the screen; 

 but as soon as the prism is placed in the path of the 

 beam, the latter is deflected, and the bright spot appears 

 at b instead of at a. If the screen is l cm from the prism, 

 the latter having a refracting angle of 10 and its 

 substance being water, the deviation of the beam 

 measured by a straight line from a to b will be found 

 to be about 5 cm *8; if the angle is 20, the distance from 

 a to b is about ll cm< l : and if the refracting substance 

 were glass instead of water, these distances would be 

 half as much again, the refracting angles remaining 

 equal. 



The laws which regulate the deviation of light by 

 prisms are somewhat too complex to be discussed in this 

 work. The deviation is not strictly proportional to the 

 refracting angle, and is less nearly so the greater this 

 angle. If one of two prisms has a refracting angle which 

 is double that of the other, the deviation produced by the 

 former will be more than double that of the prism with 

 the smaller angle. The deviation, moreover, does not 

 depend merely upon the substance and refracting angle 

 of a prism, but also on the relative direction which the 

 incident ray has to the first refracting surface ; in other 

 words, the deviation changes with the position of the 

 prism relatively to the incident light. If, as assumed in 

 fig. 261, the incident and the emergent ray make equal 



