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INTERFERENCE OF LIGHT 331 second method described. In this case a fresh film should always be taken up after all arrangements are made, as the bands are most numerous when the film is thickest. They will be seen to shift as the film thins, but care must be taken to avoid a current of air on the film as far as possible. The colours of a film of condensed vapour are readily pro- jected according to Eeade's method. A plate of glass is rubbed with soap, and then nearly cleaned off with a chamois leather; the plate is then adjusted in the Bunsen holder. Then taking a piece of rubber tube about a foot long and |-inch in bore, this is gently breathed or blown through, with the othor end directed against the centre of the soaped side, which is projected like the soap film. Coloured rings will appear on the plate. One precaution is necessary, however: as the experiment depends upon the condensation of the pio 1S2 ._ AnaIysIsotEiDg3 breath by cold, an alum trough must be placed to intercept the heat of the lantern, and the plate must be cold when placed in position. Any piece of tolerably flat iridescent glass may also be projected. That light is destroyed by interference, even with films too thick to exhibit colour, may be shown by analysing the light from a slit, as in the analysis of the soap film, from the film of air between two pieces of plate glass pressed but not rubbed together ; or by carefully splitting a very thin film of mica, and bending it round a blackened convex surface ; the reflection from this will itself give a slit, or rather a line of light, if placed in the parallel beam. The spectrum will be crossed by a number of dark parallel lines; more and thinner.