6 io EFFECT OF APERTURE IN PROJECTION [Cn. XIV 



2. It lets through more light. 



The effect of increasing the aperture of a lens when using 

 directed light as with the magic lantern depends somewhat upon 

 circumstances. If the directed light spreading out in the form of a 

 cone has a greater diameter than that of the lens, the larger the 

 lens up to the full diameter of the cone, the greater the amount of 

 light which gets through, and the brighter will be the screen image 

 just as with undirected light. 



If however, directed light from a point in the object spreads out 

 over a cone which has a smaller diameter than the lens, then the 

 size of the lens is immaterial, for all the light which gets through 

 the lens, gets through a small part of its area, and the rest of the 

 lens is not used at all. Increasing the diameter of the lens will not 

 increase the brightness of the screen image. 



Another method of looking at the problem is to suppose the 

 diameter of the lens fixed and that of the cone of directed light to 

 be increased in diameter, assuming the light source to have the 

 same intrinsic brilliancy, i. e., same brightness per square centi- 

 meter. Suppose we start with a very small source of light behind 

 the pinhole in the screen 5, fig. 343, This light will get through a 

 small part of the lens. Now increase the area of the light source L, 

 keeping everything else the same, the cone of light from 5, will for 

 a time all get through the lens and be collected at a point, hence the 

 image 7, of the pin hole will keep getting brighter until the cone of 

 light just fills the aperture of the lens. When this occurs a further 



FIG. 343. SIZE OF THE LIGHT SOURCE AND BRILLIANCY OF THE SCREEN IMAGE. 



Up to a certain point the larger the light source L, the greater will be the 

 amount of light from the specimen 5, which gets through the objective O, but 

 beyond this point increase in the size of the light source produces no further 

 increase in the intensity of the screen image as the light (u, v) passes outside 

 the objective. 



