282 OUTPOSTS OF THE INTELLIGENCE SERVICE 



distant object, is brought to bear on something near the eye, it is 

 proof that tlic main change of curvature is that of the front of the 

 lens. In acconniiodation, the central image becomes smaller and 

 moves towards the other erect image. (There are other theories 

 of the mechanism of accommodation, but the above has, at present, 

 the greatest weight of evidence in its favour.) (Fig. 72.) 



The iris corresponds to the stop or diaphragm of the camera and 

 is used for the same ends. The way in which the size of the aper- 

 ture, the pupil, is altered has been considered. It is controlled 

 from the retina, light falling on the retina producing constriction. 

 The function of the iris in accommodation is to increase the depth 

 of focus of the eye. The depth of focus in the case of the eye is the 

 greatest distance through which a luminous point can be moved 

 and still produce a sharp point image {i.e. falls on one cone) on 

 the retina. For example, a point may be seen clearly at, say, 

 100 metres, and at 0-5 metre, i.e. the depth of focus, in this case, 

 would be 100 — 0-5 = 99-5 metres. Now in any lens system the 

 depth of focus may be modified by the size of the aperture in front 

 of the lens. The depth increases as the diameter of the aperture 

 decreases. In other words, when we are near an object we use a 

 small stop so that light which might cause the image to spread 

 itself over the retina is shut out, i.e. we get definition. 



How Optical Defects are Prevented in the Eye. 



At the beginning of this chapter we mentioned some of the 

 commoner faults found in lens cameras, e.g. spherical and chro- 

 matic aberration, comma, halation, flare, irradiation and scattered 

 light, and we discussed methods of overcoming these defects {q.v.). 



Spherical Aberration. In ordinary circumstances this aberration 

 of the eye is negligible, due {a) to the special structure of the lens 

 whereby the rays are refracted in exactly the opposite way from 

 that produced by an ordinary biconvex lens ; that is, the rays 

 passing through the nucleus are more highly refracted than the 

 rays falling on the peripheral parts of the lens, and (6) to the 

 stopping action of the iris. 



Curvature of the Field. This is avoided completely by the 

 curved screen on which the image is formed. The focal length 

 of the eye (15-5 mm.), being somewhat longer than the radius of 

 curvature of the retina (10 mm.), gives an optical system practically 

 perfect in this respect. 



Chromatic Aberration. Experiment shows that the curvature 

 of the anterior surface of the lens and the diameter of the pupil are 

 so adjusted that the image is formed on the retina by the com- 

 ponent rays of white light Dresent in greatest intensity. For 



