520 THE HUMAN BODY. 



separated do not, however, excite in us visual sensations when 

 they fall into the eye, but only certain middle ones. If solar 

 light were used, with the prism, Fig. 146, certain least re- 

 fracted rays between r and 8' would not be seen, nor the 

 most refracted between v and S', while between v and r 

 would stretch a luminous band exciting in us the series of 

 color sensations from red (due to the least refracted visible 

 rays), through orange, yellow, green, bright blue, and indigo, to 

 violet, which latter is the sensation aroused by the most re- 

 frangible visible rays. The still shorter waves beyond the 

 violet can only be seen under special conditions; 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 compose 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 ourselves 

 therefore to such converging lenses. If simple light from a 

 point A, Fig. 140, fall on such a lens its rays, emerging on 

 the other side, will take new directions after refraction and 

 meet anew at a point, #, after which they again diverge. If 

 a screen, r r, be held at a it will therefore receive an image 

 of the luminous point A. For every converging lens there 

 is such a point behind it at which the rays from a given point 

 in front of it meet: the point of meeting is called the conju- 

 gate focus of the point from which the rays start. If instead 

 of a luminous point a luminous object be placed in front of 

 the lens an image of the object will be formed at a certain 

 distance behind it, for all rays proceeding from one point of 

 the object will meet in the conjugate focus of that point be- 

 hind. The image is inverted, as can be readily seen from 



Fig. 147. All rays from the point 

 A of the object meet at the point 

 a of the image; those from B at 6, 

 and those from intermediate" points 

 at intermediate positions. If the 



sin ' le lens were re p ]aced b y sev - 



converging lens. era l combined so as to form an 



optical system the general result would be the same, provided 

 the system were thicker in the centre than at the periphery. 



