LIGHT: REFLECTION 277 



With the arc this will be very sharp and clear. Parallelism is 

 of course obtained by pushing the arc-light or the lime forward 

 into the principal focus of the condensers ; or with a Duboscq 

 lantern by pushing in the condensers so that the arc is in 

 their focus. 



The lime-light does not give such an accurately parallel 

 beam, owing to the greater size of the radiant ; there is more 

 or less divergence from the aperture, so that nothing like a 

 sharp circular spot (in the case of a circular pencil) would 

 appear on the screen. Should the nozzle in work have an 

 ordinary slide stage behind it, then by placing in this also 

 (whether it be the stage of the optical front or the ordinary 

 stage of the lantern) a slit, or circular aperture, as the case 

 may be, rather larger than that on the front of the nozzle, 

 the pencil will be sharpened considerably, and rendered very 

 nearly parallel. We may call this the sharpened parallel 

 beam. 



But the mere parallel beam is not sufficient for all cases. 

 When a figure has to be produced on the screen by the motion 

 of a luminous spot, that spot must be sharp and denned, if 

 the figure is to be so. Circular pencils are always employed 

 in these cases, and should first be rendered approximately 

 parallel, by adjusting the light in the focus of the condensers. 

 If then the detached focussing lens be adjusted to focus the 

 aperture on the screen, we have all the necessary conditions. 

 The beam itself is as nearly parallel as can be, and the little 

 light that is scattered by divergence is brought back and 

 utilised by the lens, and all sharply focussed. The long-focus 

 lens, if one is at command, usually produces the best results. 

 We will call this arrangement the focussed parallel beam. An 

 arrangement for greatly increasing the brilliance of such 

 smaller beams and pencils has been shown in fig. 95, p. 173. 



155. Angular Motion Doubled by Reflection. It will 

 readily be seen on being pointed out, that any angle through 

 which the mirror is turned is doubled by the angular motion 



