THE EYE AS AN OPTICAL INSTRUMENT 221 



mixed light when sent through a prism is spread out, and decom- 

 posed into its simple constituents. For let ax (Fig. 85) be a ray 

 of mixed light composed of a set of short and a set of long ethereal 

 waves. When it falls on the surface AB of the prism, that por- 

 tion which enters will be refracted towards the normal ED, but 

 the short waves more than the longer. Hence the former will 

 take the direction xy, and the latter the direction xz. On emerg- 

 ing from the prism both rays will again be refracted, but now 

 from the normals Fy and Gz, since the light is passing from a 

 more to a less refracting medium. Again the ray xy, made up 

 of shorter waves, will be most deviated, as in the direction yv, 

 and the long waves less, in the direction zr. If a screen were put 

 at SS', we would receive on it at separate points, v and r, the two 

 simple lights which were mixed together in the compound inci- 

 dent ray ax. Such a separation of light-rays is called dispersion. 



Ordinary white light, such as that of the sun, is composed of 

 ethereal vibrations of every rate, mixed together. When such 

 light is sent through a prism it gives a continuous band of light- 

 rays, known as the solar spectrum, reaching from the least refracted 

 to the most refracted and shortest waves. The exceptions to this 

 statement due to Frauenhofer's lines (see Physics) are unessential 

 for our present purpose. Not all the rays of the solar spectrum 

 are visible to the human eye. The least refracted ones, called 

 the ultra red, and the most refracted ones, the ultra violet, do not 

 stimulate the retina; they are determined by their physical and 

 chemical effects. The visible spectrum includes in order of in- 

 creasing refrangibility the seven spectral colors red, orange, 

 yellow, green, blue, indigo, and violet. These merge insensibly 

 into one another, showing the sun's light to be a mixture of all 

 possible wave lengths, and not of certain selected ones. 



Refraction of Light by Lenses. In the eye the refracting 

 media have the form of lenses thicker in the center than towards 

 the periphery; and we may here confine ourselves, therefore, to 

 such convex lenses. If simple light from a point A, Fig. 79, 

 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 again diverge. If a screen, rr, be held at a it will 

 therefore receive an image of the luminous point A. For every 

 convex lens there is such a point behind it at which the rays 



