SOUND 257 



and still more so in projecting compound figures to be pre- 

 sently described. 



141. Management of Acoustic Pencils of Rays. -When 

 the light is brilliant and the pencil nearly parallel before 

 focussing, as from an electric lantern, it is generally more con- 

 venient to first focus the spot on the screen, and then adjust 

 the forks or other vibrating apparatus beyond the lens, or 

 between it and the screen. There will be abundance of light, 

 more room for adjusting apparatus, and the figure will be 

 projected of the greatest size, which with forks is desirable. 



But in projecting reeds as presently described, the greatest 

 size may be undesirable ; or light may be deficient. In the 

 latter case, more light can be passed through a small aperture 

 by somewhat converging the rays upon it, which diverge 

 again. Suppose we use a pencil thus diverging somewhat 

 from a inch aperture (indeed there is always considerable 

 ' scattering ' when any but the arc-light is employed) and we 

 are employing mirrors f inch diameter, which is a good 

 average size. It is possible to bring the mirror of the first 

 reed up near the aperture, so that it receives the entire pencil, 

 and even the second mirror loses little of it ; and even if the 

 plane-mirror has to be used in addition to ' analyse ' the 

 result as in showing ' beats ' there will be room for all this 

 within the conjugate focus of a suitable lens, let us suppose 

 one of 6 in. diameter and 14 to 18 inches focus. The focus- 

 sing lens is thus placed beyond the apparatus, and next the 

 screen. In such a case the result may be more brilliant thug 

 arranged. 



But there is a point to be remembered. The reflected pen- 

 cils diverge by the vibration of the mirrors ; and the focussing 

 Inns acts upon these divergencies the same as upon others. 

 The effect is to reduce the size of the figure on the screen ; 

 which may be either an advantage, or disadvantage. If the 

 lens focussed the actual vibrating mirror, there would be no 

 figure at all, focussed in this way ; but inasmuch as the lens 



