12 



THE OPTICAL DEFECTS OF THE EYE. 



size of the image depends upon its dit^tance behind the aperture; 

 if the image is as far behind thj aperture, as the object is in front, 

 the image will be of the same size as the object, if .half the distance, 

 half the size, as seen at f, g. 



■ If, in the above experiment, the aperture be enlarged, it will be 

 found that the image at the back of the box will become mucli less 

 distinct; the more the aperture is enlarged, the more indistinct will 

 be the image. The reason of this indistinctness in the image is that, 

 when the aperture is enlarged, a number of diverging rays from one 

 point in the flame pass through the aperture, and each one repeats 

 the image, so that the parts of the image overlap each other. 



This is shown in Pig. 6. A, B, represents the flame of the lamp, 

 and C, E, D, F, the image behind an aperture. The aperture is 

 supposed to be just large enough to admit two divergent rays, each 

 of these rays produces a separate image ; thus, the point A is re- 

 peated twice at D and F, and the point B is repeated at C and E. 

 The larger the aperture, the more light is admitted, but the more 

 indistinct is the image. 



If now, a convex lens be inserted in the enlarged aperture, these 

 divergent rays that enter the aperture (from every point of the 

 object) are converged to a focus ; thus in 



Fig. 7. 



