12 AMES, PROCTOR AND AMES. 



The image of point source object A upon which the eye is focused is 

 imaged on the retina at (a), that of B behind the retina at (b), that of 

 C in front of the retina at (c) . Where the image bundle which forms 

 the image (b) cuts the retina it is a cone of considerable size. The 

 object B will appear as a diffusion circle. The image bundle which 

 focuses down to the image C in front of the retina is spread out again 

 and also appears as a diffusion circle. If instead of a point source the 

 objects at B and C are objects with edges the edges will have the well 

 known appearance that is seen on an object that is out of focus, i.e., 

 a pencil held near the eye while the eye is focused on a distant object. 

 The nearer the objects B and C approach A the smaller will be the 

 size of these diffusion circles and the more similar all their images be- 

 come. The magnitude of this effect depends upon the focal length of 

 the lens system and the size of the aperture. In the eye this means, 

 broadly speaking, the length of the eye and the size of the pupil. It 

 is very marked where either the objects in or out of focus are close to 

 the eye but decreases as they are moved away and becomes im- 

 perceptible when they all are at a distance of thirty feet or more. 



CHROMATIC ABERRATION. 



As was described in Chapter I the effect of chromatic aberration is 

 to cause light of different wave length or color to focus at different 

 distances from the lens. See Figure 2. 



If we move the blue point source at A, Figure 2, towards the eye, 

 the eye being kept focused on A, it will cause the image of the blue 

 source to move back towards the retina. A position B, Figure 10, 

 will be found where a blue light will focus sharply on the retina while 

 the eye is still focused on the point at A. If we move the red light 

 away from the eye a similar position R, Figure 10, will be found where 

 the red light will also be in focus. 



Figure 10. Diagram showing how images from sources of different colors 

 situated at different distances from the eye can all be in focus at the same time. 



That is all three lights will be in focus at the same time although 

 they are at very different distances from the eye. For example if the 

 eye is focused on a yellow light at a distance of six feet it will see 



