STIMULATION THE SENSES 115 



the cornea. This can easily be seen by a simple experiment on the 

 eye of an albino rabbit (E., p. 213). But remembrance of the fact 

 that the liquid in the eye has a higher refractive index than air, and 

 that it is bounded by a spherical surface, is sufficient to bring 

 conviction. The material of the lens itself has a refractive index 

 not much higher than that of the liquid in which it lies, so that the 

 actual refraction due to it is not great. What it does is to adjust 

 the focal length of the dioptric system of the eye, so that sharp 

 images of objects at various distances from the eye may be formed 

 on the retina. This it does by altering its curvature. The greater 

 the curvature of a refracting surface, the shorter its focus. Many 

 photographic lenses are double, so that each part can be used 

 separately. One part is often of shorter focus than the other, and 

 can easily be seen to have a more curved surface. The mechanism 

 by which accommodation to objects at different distances is effected 

 in the case of the eye of the higher vertebrates is, briefly, as follows : 

 The lens is an elastic body, which has, when released from its 

 position in the eye, a particular natural curvature. In its normal 

 position in the eye it is pulled flatter by the way in which it is held 

 stretched between membranes in front of it and behind it, which 

 are kept in a state of tension. There is, further, a ring of muscle, 

 the ciliary muscle, whose fibres are arranged in such a way that 

 when they contract they pull the place to which the suspension of 

 the lens is attached nearer to the lens itself, and thus lessen the 

 tension on it, allowing it to approximate more or less to its natural 

 curvature. Its focal length is diminished, and the image of a near 

 object, which would otherwise be formed beyond the retina, is thus 

 brought to lie nearer to the lens and on the retina itself. 



This is not, of course, the way in which the photographer adjusts 

 the focus of his camera ; he moves the lens backwards and forwards, 

 since its curvature is fixed. In some of the lower animals, indeed, 

 a means of accommodation like that of the camera is adopted, a 

 muscle being present to change the distance of the lens from the 

 retina. 



There are two further arrangements common to the camera and 

 to the eye. The diaphragm, which enables sharper images to be 

 formed by limiting the part of the lens used to the middle, naturally 

 with loss of light, is represented by the iris, the coloured screen 

 with the aperture, the pupil, in front of the lens. The iris contains 

 muscular fibres arranged in a radial direction, which enlarge the 

 pupil when they contract, and others in a circular direction, which 

 narrow it. In the eye, however, the chief use of the iris is to 

 prevent excess of light from reaching the retina, and the improve- 

 ment in sharpness of vision is secondary, although advantageous 

 when the light is strong enough to permit it. 



