530 PHYSIOLOGY 



tin- pupil Ilirou^h which the light passes. In the caseoi any 1ms system 

 such as the eye, in which the pupil is circular, experiment and calculation 

 are agreed that the image of a point source consists of a series of concentric 

 rings of light, having a bright spot at their centre. The diameter of thi 

 spot in the case of the eye is found to be 0*01 mm. with a pupil of 2 mm. 

 diameter. No matter how perfect the eye be as an optical instrument, diffrac- 

 lion sets a limit in this way to the perfection of the image that can bo formed. 

 This should not however be thought of as a defect but as a property, since 

 it is caused by the nature of light itself. In the consideration of the principal 

 optical errors of the normal eye we have to decide in each case not only 

 to what extent the defect is present, but whether the defect produces any 

 noticeable change in the diffraction pattern which may affect definition. 



DEPTH OF FOCUS, like diffraction, is a property of a lens (system and not an 

 aberration. Its origin may be explained as follows: Suppose objects 100 metres 

 away to be forming sharp images on the retina, then objects at 200 metres will form 

 images which come to a focus slightly in front of the retina, and objects at 50 metres 

 images that are slightly behind. If however the focussing points are only a short 

 distance in front of or behind the retina, the image of a distant point which fell on ;i 

 single cone would still do so because the cone has a certain diameter, although its dist mi-r 

 from the eye had been altered. 'Depth of focus in the case of the eye is the greatest 

 distance through which a point can be moved, and still produce an image which falls 

 exactly on a cone without spreading at all on to neighbouring ones. For example, 

 in the above case the distance moved was from 200 to 50 metres, that i -. the depth of 

 focus was 150 metres. Now it is found in the case of any lens system that depth varies 

 with the aperture of the pupil. Thus in the case of the eye the following values are 

 obtained. 



Pupil <li;im !-. l ptli at infinity. h pthat5cm& 



1 mm. . . From inf. to 8 metres . . . . 3-2 em. 



2 16 1-6 ,, 



3 ,, . . ,, ,, 24 ,, . . . . 1 ! ,, 



4 32 -8 

 We see therefore that not only does depth decrease as the aperture of the pupil 



increases, but that it also decreases as the mean distance of the objects from the eye 

 decreases. Thus with a pupil of 3 mm. the eye if focussed sharply on objects '24 metres 

 away, would also be in focus for objects at infinity and also for objects at 12 metres. 

 Depth of focus is therefore considerable at this distance. But if the eye is working at 

 the ordinary reading distance (25cm.)depth would be 1-1 cm. only. Ata pupil diameter 

 of 1 mm. depth would be increased threefold, and therefore the closure of the pupil, 

 which accompanies accommodation and convergence for near objects, has the valuable 

 pmprity of increasing the depth of focus at the same time. 



CHROMATIC ABERRATION OF THE EYE. It was shown in Section I 

 that whit r liirht consists of a number of rays of different wavelength, 

 and that the short rays on refraction are more bent than the long. 

 When therefore white light is incident on a lens, the rays of short 

 wavelength come to a focus in front of those of longer wavelength. This 

 difference of focus for rays of different colour is called chromatic difference 

 of focus. Experiment shows that, when such a series of foci are formed 

 by the eye, the accommodation is so adjusted that the rays of greatest 

 intensity (usually yellow rays) form the most sharply focussed image, and 

 the colours of longer and shorter focus form blurred discs of light of 



