324 OPTICAL PROJECTION 



destroyed ; but when so placed that crest is superposed on 

 crest, there is a wave of double the height. Every arrange- 

 ment is projected on the screen quite sharply by the shadow 

 method ( 109). The balls at the top of a Powell's wave 

 apparatus are easily projected in the same way. 



185. Thin Films. The readiest methods of demonstrating 

 the interference of light are by reflection from the two surfaces 

 of thin films. When the lantern is worked upon a table, a 

 small black tray about 12 inches diameter may be partly filled 

 with water, may be laid down in front of it, and the lantern 

 either canted up behind so that the parallel beam from the 

 flange nozzle comes down at an angle upon the water and is 

 thence reflected rather upwards to the screen ; or the long- 

 focus lens may roughly focus the surface upon the screen, 

 being adjusted in the rising reflected rays ; 

 or the beam may be deflected downwards 

 from the nozzle by the plane mirror, and 

 received and re-deflected in their upward 

 path by another plane mirror, the surface 

 being focussed. Then dipping a rod in oil 

 FIG. i77.-Fiim of oxide of turpentine, and letting a drop fall upon 

 the water, it rapidly spreads into a thin film, 

 which produces beautiful colours upon the screen. 



A wide-mouthed goblet of water may be placed in the 

 phoneidoscope apparatus shown in fig. 142, the surface of the 

 water occupying the place of the soap-film. Only very thin 

 and fresh oil of turpentine will show colours readily in a 

 confined space like the goblet, the glass preventing the 

 spreading of the film sufficiently ; but benzol containing a 

 very small quantity of Canada balsam or turpentine in solution 

 will act very well in such an apparatus. 



Thin films of oxide show the same phenomena very readily, 

 placing a polished steel plate on a light tripod with a spirit- 

 lamp underneath, in the apparatus shown at fig. 142. If the 

 polished plate is first slightly warmed uniformly, and smeared 



