LIGHT: COLOUR 285 



s s. All being adjusted, the double prism, p and p 2 , is turned 

 round the perpendicular axis till all the rays pass through to the 

 prism B, which throws its spectrum as usual. Let them now 

 be very slowly turned in the direction of the hands of a watch. 

 Then, just as the film of air arrives at the ' critical angle,' as 

 it is called, the violet rays being totally reflected leave the 

 spectrum s s and appear at s s. On continuing to turn, all the 

 colours in succession do the same. 



The fact that lenses disperse light in the same way, 

 convex lenses bringing blue rays to a nearer focus than red 

 rays, is very readily demonstrated by placing in the stage a 

 piece of black card with two apertures pretty close together 

 in a horizontal line, one covered with deep red, and the other 

 with blue gelatine, and focussing the parallel beam sent 

 through them by the top edge of the loose lens. The two 

 images will not both focus on the screen at the same time, or 

 at the same level. 



163. Composition of White Light. Colour a Shadow, or 

 Suppression. These two connected facts may be shown by 

 numerous experi 

 ments. Arrange as N A \ 



for the simple spec- " 

 trum, but not deflect- 

 ing the lantern from 

 the screen. The 

 prism A being duly 



placed before the nozzle N, or in front of the loose lens if 

 that is employed, place in front of it a second prism, or: 

 bottle B, with its refracting angle turned the other way. 

 This second prism gathers up again all the coloured rays, and 

 recomposes a white image of the slit on s. If we now introduce 

 a black card c at one side (the second prism being placed as 

 far from the first as will gather up all the rays, and the 

 card kept close to the second one), we suppress, or stop off 

 part of the colours, and the image of the slit at once becomes 



