6 AMES, PROCTOR AND AMES. 



Chapter I. 

 DISTINCT VISION. 



Distinct vision deals with the nature of retinal images of objects at 

 which the eye is looking directly, i.e., that are in sharp focus. As we 

 have a very definite conception of the appearance of objects at which 

 we look directly it may hardly seem necessary to analyze the char- 

 acteristics of the images of such objects. It is believed however that 

 such an analysis will not only be instructive but will make it easier to 

 understand the characteristics of the images of objects upon which the 

 eye is not focused, which will be taken up later. 



Owing to the nature of the lens system of the eye the image on the 

 retina formed by an object at which the eye is directly looking is not 

 an exact copy of the object itself. The image is spread out and some- 

 what diffused. This is due primarily to three factors; chromatic aber- 

 ration; spherical aberration; and irregular astigmatism. These factors 

 and their effects will be considered in the order given. 



CHROMATIC ABERRATION. 



Chromatic aberration causes light of different wave length or color 

 to come to focus at different distances behind the lens, light of shorter 

 wave length, i.e., towards the blue end of the spectrum focusing 

 nearer the lens. This is shown in Figure 2. 



Figure 2. Diagram showing chromatic aberration of the eye. 



If we have three point sources of light at A, one red, one yellow, and 

 one blue, the image of the blue source will be formed at (b) the image 

 of the yellow source at (y), while the red image will be at (r). Figure 

 3 shows the shape of image bundles as formed in Mr. Ames' eye by 

 point sources of red, yellow and blue light. These of course are much 

 enlarged ; the lens system of the eye is to the left. 



It will be seen that the blue bundle lies to the left or nearer the lens 



