LIGHT. 8l 



so adjusted as to have it vibrate in a perpendicular 

 plane like m' . This can be done by fixing its hinge at 

 right angles, and the rest the same as for mirror m'. 

 The reflected beam from the second mirror may be 

 received upon a large mirror held in the hands, and 

 thence reflected upon the wall or screen. 



LIGHT. 



RECTALINEAR MOVEMENT. 



That light moves in straight lines can be shown by 

 admitting the light from the porte lumiere through a 

 small hole. It goes straight across the room, and its 

 course can be tracked through the room by the dust 

 particles, or a little smoke, which it will light up. Also, 

 by having the room otherwise quite dark, permit the 

 light to come in the round orifice, half an inch in diam- 

 eter, as it is reflected from the landscape outside, and 

 not reflected from the mirror. The room is thus a large 

 camera obseura, and an inverted image of the landscape 

 will be seen upon the walls, or upon a small screen held 

 a foot or two from the orifice. This image will be par- 

 ticularly strong if the ground be covered with snow, as 

 much more light is reflected from that than from grass 

 or foliage. If persons are passing their forms will be 

 seen, and appear as if walking head downward. 



Parallel rays A will be reflected from the mirror of 



