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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