H2$ OPTICAL PROJECTION 



Sometimes the heat of the lantern will destroy the film 

 too rapidly. In that case an alum cell in the slide-stage will 

 remove the difficulty. 



Horizontal bands being caused by the action of gravitation 

 on the film, if we can make the gradual thinning take a 

 circular form, the bands will become annular. This is 

 effected by a beautiful modification of the experiment often 

 employed by Lord Eayleigh, and which is very easy if any 

 sort of bellows, or weighted bag filled with air, be at hand, 

 to furnish a slight blast. Even the breath will answer, but 

 needs some practice and a thin rubber balloon to steady the 

 pressure. Connected with the air-blast (a gentle one) by 

 rubber tubing is a piece of glass tube drawn into a small but 



A B 



FIG. 180. Rotating Films 



not capillary orifice : one of the ' fillers ' sold for use with 

 fountain pens answers excellently. This is fixed in another 

 small Bunsen holder, and adjusted so as to direct a blast at a 

 small angle with the surface of the film. Adjusted near the 

 edge as at A (fig. 180) the above is converted into a single 

 whirling vortex, which shows rapid and magnificent gradation 

 of colour : adjusted as at B, a little to one side of the centre, 

 the stream divides into two vortices, in which the play of 

 colour is still more rapid. This experiment never fails to 

 'bring down the house.' 



187. Films of air Newton's Rings. Two squares of plate 

 glass, with rounded edges, cleaned and polished, are readily 



