290 OPTICAL PROJECTION 



paper s, with a similar aperture cut in that. It must not be 

 forgotten to adjust a small black screen behind the bulb B, to 

 prevent any direct rays from incommoding the audience and 

 spoiling the effect. By the refraction, reflection, and disper- 

 sion of the bulb, a prismatic circle B will be formed upon the 

 screen, 1 and it will be evident that wherever a colour say blue 



Fia. 157. Rainbow Experiments 



reaches the screen, an eye whose pupil was at that spot, or 

 in its precise direction, would see blue from that particular 

 drop of rain represented by the bulb. 



165. Variation in Dispersive Power.This may be shown 



1 This experiment cannot be performed with less than the lime-light ; but 

 if any difficulty be found with that, it will be easily surmounted by using the 

 focuased parallel beam (p. 277) and the attachment described on p. 173. 



