438 THE OUTGO OF ENERGY 



THE SCHEMATIC EYE 



In passing from the external air to the retina, 

 the rays of light undergo refraction at the layer 

 of tears on the anterior surface of the cornea, the 

 surfaces bounding layers of unequal refractive 

 power in the substance of the cornea, the ante- 

 rior surface of the aqueous humor, the anterior 

 surface of the lens, the surfaces bounding layers 

 of unequal refractive power in the substance of 

 the lens, and the anterior surface of the vitreous 

 humor. To determine the size and position of 

 visual images, it is fortunately not necessary to 

 calculate refraction at each of these many sur- 

 faces. The problem is much simplified by the 

 following considerations. 



The irregularities in the refractive power of 

 the different parts of the cornea are small ; and 

 the refractive index of the layer of tears which 

 covers the anterior surface is almost identical 

 with the index of the substance of the cornea 

 and that of the aqueous humor. Practically 

 therefore the layer of tears, the cornea, and the 

 aqueous humor may be regarded as a single re- 

 fracting medium. Further, although the layers 

 of which the lens is composed increase in refract- 

 ing power towards the centre ,of the lens, it is 



