270 OPTICAL PROJECTION 



mounted independently as in fig. 141, KK being the capsule, 

 a the sound tube, b the gas-tube, and c the flame. This must 

 be shaded in all but the one direction as before, and the 

 focussing lens brought next to it, whilst the revolving mirror 

 comes on the other side of the lens. The flame is thus 

 shielded from the draught of the revolving mirror, and this 

 latter receives the rays in a more favourable manner. The 

 gain in brilliance is very marked, and with ' enriched ' gas 

 very fine projections will be obtained. 



With a revolving mirror of double the usual height 

 in fact sufficiently tall to embrace the usual three flames on 

 an organ-pipe for showing nodes, the projection method has 

 an advantage over the direct. Using a focussing lens for each 

 flame, 'ihe lenses may be easily adjusted to somewhat converge 

 the images, so that they may be brought as close together upon 

 the screen for comparison as may be desired. 



A concave spherical mirror of about three feet focus, 

 rotated on a vertical axis, will also give detached images, 

 but the revolving mirror is cheaper and more convenient. 

 The best and most brilliant method of all, however, is that 

 devised by Mr. C. V. Boys, who projects such phenomena 

 through a circle of lenses fixed round the edge of a revolving 

 disc. 



Sensitive flames are easily projected in the same way with 

 enriched gas. 



Tyndall's sensitive smoke-jets are very effective projected 

 by the shad6w method much more so than viewed direct, 

 the smoke-shadows being very distinct upon the screen. 



148. The Phoneidoscope. Mr. Sedley Taylor's phoneido- 

 scopic figures may be projected in various ways. A soap-film 

 may be taken up even on the end of a lamp-chimney, and if 

 arranged so as to be projected, will show figures under the 

 influence of sonorous vibration. Or the ordinary phoneidoscope 

 may be used, reflecting a converging beam from the condensers 

 down upon the film by the plane mirror, and focussing upon 



