REFRACTION BY LENSES. 495 



the long waves less, in the direction z r. If a screen were 

 put at S S' , we would receive on it at separate points, v and 

 r, the two simple lights which were mixed together in the 

 compound incident ray a x. Such a separation of light-rays 

 is called dispersion. 



Ordinary white light, such as that of the sun, is com- 

 posed of ethereal vibrations of every rate, mixed together. 

 When such light is sent through a prism it gives a contin- 

 uous band of light-rays, known as the solar spectrum, reach- 

 ing from the least refracted to the must refracted and short- 

 est waves. The exceptions to this statement due to Frauen- 

 hofer's lines (see Physics) are unessential for our present 

 purpose. All of the simple lights into which the compound 

 solar light is thus separated do not, however, excite in us 

 visual sensations when they fall into the eye, but only cer- 

 tain middle ones. If solar light were used with the prism. 

 Fig. 128, certain least refracted rays between r and 8' would 

 not be seen, nor the most refracted between v and 8 ; while 

 between v and r would stretch a luminous band exciting in 

 us the series of colors from red (due to the least refracted 

 visible rays),, through orange, yellow, green, bright blue, 

 arid indigo, to violet, which latter is the sensation aroused 

 by the most refrangible visible rays. The still shorter 

 waves beyond the violet can only be seen under special con- 

 ditions; they are known mainly by their chemical effects 

 and are called the actinic rays; the invisible waves beyond 

 the red exert a powerful heating influence and compr.se the 

 dark heat rays. The eye, as an organ for making known 

 to us the existence of ethereal vibrations, has, therefore, 

 only a limited range. 



Refraction of Light by Lenses. In the eye the refract- 

 ing media have the form of lenses thicker in the centre 

 than towards the periphery; and we may here confine our- 

 selves therefore to such converging lenses. If simple light 

 from a point A, Fig. 122, fall on such a lens its rays, 

 emerging on the other side, will take new directions after 

 refraction and meet anew at a point, a, after which they ngain 

 diverge. If a screen, r r, be held at a it will therefore 

 receive an image of the luminous point A. For every con- 



