THE REFRACTION OF THE EYE 533 



and magnification. In addition however we must take into account the very important 

 effects of diffraction. The final results of such a calculation show that 

 che images of rays of different wavelength overlap one another. At the 

 centre of the image is seen the sharp yellow focus of the highest intensity. 

 Eccentric to it and overlapping one another are seen the diffuse red and green 

 foci, which are of much less intensity. Where these overlap they produce a 

 compound yellow according to the rules of colour mixture. Further outwards is 

 the still more diffuse image of the blue rays, which is of almost negligible intensity. 

 It is seen therefore that the centre of the image is entirely occupied by the sharp and 

 intense focus of the yellow rays. Not only are these rays the brightest in the spec- 

 trum, but they are also those nearest to white light in their physiological properties. 

 It is because of this structure of the image that the acuity of vision is so great at the 

 fovea. 



PERIPHERAL IMAGES have, as stated above, been to a considerable extent sacri- 

 ficed, so far as their definition is concerned, in order to obtain the best possible conditions 

 at the fovea. We find therefore at the periphery, images that in no way compare 

 with those formed near the optical axis of the eye. Even here however there is 

 evidence that the eye has been designed to give the best results obtainable. The two 

 aberrations which most concern us other than those already mentioned are curvature 

 of the field and distorsion. 



CURVATURE OF THE FIELD is found in all positive lens systems of simple 

 formula. Since the photographic plate is flat it is one of the principal errors to be 

 corrected in the photographic lens. In the eye on the contrary the effects of the 

 aberration have been avoided not by correcting the lens system but by curving the 

 sensitive surface to correspond. Calculation shows that, for correction, the radius of 

 the surface of the retina should be somewhat shorter than the equivalent focal length of 

 the lens system. But we know that the radius of the retina is approximately 10 mm., 

 while the focal length of the eye is 15-5 mm. The required conditions therefore appear 

 to have been fulfilled. 



DISTORSION shows itself in photography by a curving of lines which are known 

 by experience to be straight. But this straightening, which can be readily effected by 

 suitably designing the lens system and by using a flat plate, is found to be accompanied 

 by a change in the size of an image, according as it is formed at the centre or the edge 

 of the plate. But such a change in size would be most disadvantageous in the case 

 of the eye, because not only would the apparent size of objects vary, according as their 

 images fell on the centre or the periphery of the retina, but also the perception of 

 perspective, which, as we shall see later, depends on the correct estimation of the differ- 

 ences between the position of near and distant objects, would be seriously inter- 

 fered with. In the eye it is very much more important that images should 

 keep the same size, than that distortion should be corrected by optical means. In order 

 that images shall be constant in size, the retina must be curved to approximately 

 the same extent as is required for the correction of curvature of field. We see therefore 

 that the shape of the retina has a very important effect on peripheral vision, and further 

 that, so far as we are able to judge, the best shape has been adopted. 



OTHER OPTICAL DEFECTS. The analogy between the eye and the photo- 

 graphic camera shows that there are a number of other defects from which the eye 

 may suffer ; these are (1) the presence within the eyeball of light scattered from one 

 part of the retina to another (equivalent to shiny bellows in the camera) ; (2) the 

 spreading of the image formed on one part of the retina to neighbouring portions 

 (equivalent to halation) ; (3) the illumination of the retina by light internally reflected 

 at the different optical surfaces (known in photography as flare) ; (4) the exposure 

 of parts of the retina close to those receiving the image because of imperfection in the 

 optical system (called irradiation). 



Scattered Light. In describing the histology of the retina it will be shown how 

 generously the layer of cells lying immediately under the sensitive layer of rods and 



