4 OPTICAL PROJECTION 



With each we get another image, and it will be seen how they 

 confuse each other ; while if we stop all the others each one 

 is distinct, and each of the last four is outside, on the screen, 

 that from the central hole. It will now be understood 

 instantly, that if we could bend in the rays which form the 

 four outside images, so that they would fall exactly on the 

 same spot as the central image, we should have again but 

 one and a distinct image, but five times as bright as from one 

 pinhole. 



4. Use of the Focussing Lens. To do this is the sole 

 operation of the focussing lens, in a lantern or any other form 

 of projecting apparatus. We can bend rays of light easily, 

 by sending them at an angle through the surface of some 

 other medium of greater (or less) density than the air ; and 

 the greater the angle at which the ray strikes the denser 

 medium, the more it is bent ; the amount of ' refraction ' at 

 lifferent angles being connected by a simple law which need 



not be discussed here. 

 We only need to remem- 

 ber, that on entering the 

 denser medium the ray 

 is bent in towards the 

 perpendicular, and in 

 leaving the denser me- 

 dium away from it. If, 

 then, we have a piece 

 of glass with inclined 

 faces, called a prism 

 (fig. 2), to whose faces the dotted lines N.I and N E are perpen- 

 diculars or * normals,' 1 the ray s i will be bent towards the 

 normal, to the path i E, and on leaving the prism will be 

 similarly bent away from E N to E R. Thus it is permanently 

 bent in, or rofracted, towards the thick side of the prism. 



1 All angles in optics are reckoned from the perpendiculars or normals, 

 not from the surfaces themselves. 



