INTERFERENCE OF LIGHT 329 



worked together till they show beautiful air-films. It is well 

 then to keep them together with spring wooden clips, one at 

 each corner, pinching one corner in the Bunsen holder, and 

 projecting exactly as the soap-film in fig. 179. The slightest 

 additional pinch between finger and thumb alters the colours, 

 thus leading up to Newton's experiment. 



It is not easy to get a good pair of Newton's rings, which 

 under pressure give true circular figures. The usual three 

 screws are too few, causing distortion ; six screws, with another 

 at the centre of the back glass, give better figures. If the 

 back glass is black, or some dark colour, the effect is better ; 

 the colours not being diluted by the reflected light from its 

 bottom surface. The rings are projected exactly as the soap 

 film. It is most convenient to have a stud projecting from 

 the circular frame, which fits into the socket of one of the 

 pillar- stands. 



If a wedge-prism (not too thin) be interposed between 

 the focussing lens and the screen, if will be shown that in 

 the deflected image the number of rings is very greatly in- 

 creased on one half of the image. 



By rapidly interposing in turn a red and a blue glass 

 between the lantern and the glasses, it is shown that the red 

 rings are larger than the blue ones ; but it is better to have a 

 red and blue glass framed so as each to occupy half of the 

 space in an ordinary lantern slide-frame. Holding this as 

 close as possible to the lenses, but so as to allow the reflected 

 rays to escape to the focussing lens, one half the image will 

 exhibit the red and the other the blue rings. It must be rather 

 a light blue glass. 



Having another pair of glasses, of which the lower one is 

 platinised on the surface (silvering reflects too much light, 

 and overpowers the rings), and adjusting the glasses so that 

 the light from the lantern falls on them at the polarising 

 angle, the reflection from the first or glass surface of the 

 film of air can be totally abolished by placing a Nicol prism 



