HOW \VI'; SEE. 17t) 



mind. 'V\\o .listaiu-e of an oltjcct from us gives us impor- 

 tant datu fur tV>rniing judiinient as to its size and thu size 

 of it if known, enables us to form an idea of its distance. 

 You will readily see the truth of thit^, when you know 

 that a live cent piece, held a few inches from the eye, 

 forms as lar^e a retinal imaue as the sun which is many 

 millions of miles further oil. Physiologists would tell you 

 that they both subtend the same visual angle. So if we wish 

 to have a definite idea of the actual size of an object we 

 must know its distance from us; and, on the other hand, in 

 order to judge of the distance of an object, by the size of the 

 retinal image, we must know its size as compared with other 

 objects. In the judgment of distance, however, there are 

 other factors not to be overlooked. One of the most im- 

 portant of these is what is called the " muscular sense." In 

 looking at objects close at hand we bring into play the ac- 

 commodation of the eye which, as I have already told you, 

 is brought about by the jtction of the ciliary muscle. The 

 nearer objects are to us the stronger must be the action of 

 the muscle in order that we see distinctly, and we come 

 naturally to associate a strong effort of the accommodation 

 power with nearness of the object. The amount of conver- 

 gence of the two eyes, which is necessary in order that both 

 be fixed upon the same object, also furnishes us with im- 

 portant knowledge of the distance of objects from us. 



We come now to consider one of the most remarkable at- 

 tributes of objects, a knowledge of which is given us through 

 the eye. We allude to that wonderful quality called color. 

 In the .strict sense of the word, however, no object is colored. 

 Color is a property of light, while those objects whicli aj)- 

 pear to us colored simply decompose the light falling on 

 them into its different color constituents, sending to the eye 

 some one or more and absorbing the others. For ordinary 

 purposes, however, it is more convenient to consider color 

 a.s belonging to the object, but we must bear in mind, at tlie 

 same time, that each color is represented physically by a 

 certain number of vibrations of the luminiferous ether. 

 Thus, 451 million million vibrations of the ether in a 



