8 OPTICAL PROJECTION 



simple lenses answer every purpose, and are preferable because 

 they stop less light ; some being lost at every piece of glass 

 by reflection from its surfaces, and by absorption in its 

 substance. 



For, in the third place, it will be manifest that the brightness 

 of a projection must depend upon utilising in our final image 

 a sufficient number of luminous rays. If we regard the rays 

 proceeding from an object as equally luminous in every 

 direction, the number of rays collected must obviously depend 

 solely upon the size of the lens, and its distance from the 

 object when focally adjusted : the brightness of the image will, 

 however, further depend upon the screen-distance. By a very 

 simple and obvious law, the light falling on a lens must 

 diminish as the square of its distance from the object; and that 

 on the screen, in the inverse ratio of the surface of the lens 

 and that of the superficies covered by the rays on the screen ; 

 both depending, as we have already seen, upon the focus of the 

 lens. 



Hence comes the utility of large lenses, which are highly 

 advantageous for many physical experiments in projection, as 

 we shall see. But it by no means follows that a large lens 

 will give more luminous results in every case ; for the result 

 depends upon collecting an adequate number of luminous rays, 

 and it is by no means to be taken for granted that the rays 

 emitted from an object are equally luminous in all directions : 

 it may rather be the case that all which are luminous enough 

 to afford us much help towards our final image, are confined 

 to a very limited range. To understand this is the last essential 

 point in the problem of projection. 



5. Use of a Condenser. We have already had a proof and 

 example of this. When we first removed the lens and pricked 

 a hole in the tinfoil, the margin of the image of the slide was not 

 illuminated, and we had to draw back the light considerably from 

 its usual position before we could make it so. The slide itself 

 was amply illuminated, we know, for with the lens the image 



