244 ON LIGHT. 



perceived in the phenomenon of astronomical refraction, 

 by which the sun or moon is rendered visible when 

 actually sunk below the level of the true horizon. 



(28.) From what is above stated, it is easy to see that 

 when a ray is transmitted through a sheet or plate of any 

 substance (as a window-glass) with parallel surfaces, its 

 course after emergence will be parallel to its original 

 direction, so that though displaced laterally, its direction 

 in space is unchanged, which is the reason we see objects 

 in their proper directions through a window. If the sur- 

 face at which it emerges be not parallel to that through 

 which it enters, this exact restoration of the original 

 direction will not take place ; and as we judge of the 

 situation of an object only by the direction in which its 

 light ultimately enters the eye, anything seen through a 

 transparent substance whose surfaces are so inclined, 

 will appear shifted in angular position. Any transparent 

 substance so formed of polished plane surfaces inclined 

 to each other, is called in optics "a prism;" and the 

 angle at which the two planes in question meet, or 

 would meet if extended, its " refractive angle." If such 

 a prism of glass, for instance be held before the eye 

 with its refractive angle vertical, and to the left, an 

 object seen through it will appear deviated or shifted to 

 the left of its true situation, the ray (as a slight consi- 

 deration will show) being oent towards the thicker 

 part of the prism. And thus by a very simple calcula- 

 tion, with which we shall not trouble our readers, from 

 the angular amount of deviation caused by a prism of 

 any medium whose refracting angle is measured, can 



