532 PHYSIOLOGY 



PERIPHERAL ABERRATIONS OF THE EYE. 



So far the definition of an image lying on the principal axis of the 

 lens has alone been considered. When this is not the case other conditions 

 are encountered which introduce less favourable conditions. In the first 

 place the rays that form images on the peripheral parts of the retina make 

 considerable angles with the surfaces of the eye media. This will cause 

 chromatic difference of magnification, since blue rays will be more bent 

 and will therefore form smaller images than red rays. It will also introduce 

 ' comma,' that is, the effect due to disobedience of the sine condition. It 

 is seen at once therefore that the image formation by the periphery of 

 the eye is altogether more imperfect than it is at the centre. 

 The presence of the nucleus of the lens still further impairs the marginal 

 definition. In fact we may say that in the eye, as in the microscope objective, 

 the marginal images have been sacrificed in order thereby to improve the 

 central ones. That this has been a very valuable policy will be shown later. 

 It will be shown in the next section that the most sensitive region of 

 the retina is not exactly in correspondence with the optical axis of the lens 

 system of the eye, being displaced approximately 05 mm. to the temporal 

 side. We must therefore consider briefly to what extent the peripheral 

 aberrations of which we have just spoken will interfere with the definition. 



THE SINE CONDITION (COMMA). This aberration is found to show itself in 

 optical instruments by a difference in the position of the various parts of the image 

 produced by the separate zones of the lens. Instead of rays from a point source 

 coming to a focus at one and the same point, they are found to. form a fine 

 line or comma, the tail of which points towards the optical axis. If the optical 

 system obeys a rule called the sine condition, comma is corrected. The eye appears 

 to obey this condition exactly, and therefore so far as comma is concerned the dis- 

 placement of the fovea to one side of the optical axis is no disadvantage. 



CHROMATIC DIFFERENCE OF MAGNIFICATION, like chromatic difference 

 of focus, is caused by the unequal refraction of light rays of different wave- 

 length. But since on refraction violet rays are more bent than red rays, their 

 foci form not only at different distances from the cornea, but also at different angles 

 with the optical axis. It follows from this that objects, subtending a consider- 

 able angle at the eye, produce images which are smaller for violet rays than' they 

 are for yellow rays, while those for red rays are larger still. Images produced by rays 

 of different wavelength therefore vary in size. Since that part of the retina which 

 possesses the best vision (the fovea) is situated to one side of the optical axis, all images 

 formed on it must suffer from this error. Far from this being a disadvantage however, it 

 is surprising to find that there is because of this an actual diminution of the effects of 

 chromatic aberration, and that the displacement is therefore a wholly beneficial one. 

 That this is probably the explanation of the development of the most sensitive part 

 of the retina at this p >int hardly requires indication. 



RADIAL ASTIGMATISM must, according to optical theory, be present in the 

 image formed on the fovca. Experiment shows however that its effects can be to a 

 considerable extent neutralised by positive axial astigmatism such as is found in the 

 eye of emmetropos. The presence of this aberration may therefore be ignored so far 

 as the fovea is concerned. 



STRUCTURE OF FOVEAL IMAGE may be determined approximately by con- 

 sidering in turn the effects of different aberrations on the light rays which enter the 

 eve. For this purpose the only errors of importance are chromatic differences of focus 





