THE OPTICAL DEFECTS OP THE EYE. 3 



light. The light from an object seen by moonlight is reflected twice 

 before it reaches the eye. The moon reflects the light from the sun, 

 and the object, the light which it receives from the moon. 



Every luminous object gives off", or radiates, in every direction, an 

 infinite number of straight lines of light. Each of these lines taken 

 alone is called a ray of light. A bundle of rays is called a learn of 

 light when the rays run parellel to each other. When the rays 

 diverge from a luminous point or are made to converge to a focus they 

 are called a pencil of rays, thus : 



Mg. i represents a pencil of rays diverging from a flame E, after 

 passing a convex lens they are rendered parallel and these parallel 

 rays passing the second convex lens B, the rays are converged to 

 the point (focus) P. 



The parallel rays may be called a parallel pencil ; the diverging 

 rays a divergent pencil, and the convergent rays a convergent pencil. 

 The point where rays of light meet is called th.Q focal point or simply 

 difocus. 



Strictly speaking,, there is no such thing in nature as parallel rays ; 

 the nearest approach we have to it are the rays of light we receive 

 from the sun and the fixed stars. Practically, for our purpose how- 

 ever, we may consider rays of light parallel that are received by the 

 pupil of the eye from objects that are twenty feet distant or any dis- 

 tance greater than that. Pencils of light from objects less than 

 twenty feet distant are more decidedly divergent. 



A good illustration of a divergent pencil can be obtained from a 

 lighted lamp or candle in a dark room. If a piece of card board, with 

 a small circular opening in it, be held near the lamp, you will have, 

 upon the opposite wall, an illuminated spot of the same shape as 

 the opening in the card, but very much larger. 



