May 23, 1878] 



NATURE 



87 



real friends of education, even in Yorkshire, spending as 

 much energy to provide the Government with a reason 

 for doing nothing as might itself have built and endowed 

 a college. 



PHYSICAL SCIENCE FOR ARTISTS'^ 

 III. 



LET me begin my third paper by an attempt to 

 graphically illustrate the conclusions to which I 

 drew attention at the end of my second. These con- 

 clusions were as follows : — (i) Very complex molecules 

 when in vibration give us light that we call white, which 

 light, when split up by a prism, gives us a spectrum 

 consisting of 



Red 



Orange 



Yellow 



Green "* 



Blue 



Indigo 



Violet, 



going from one end to the other. We will represent this 

 by open letters (the initials of the various colours) to 

 show that this is a case of the giving out of light. 



w a © © Y ® ^ 



We next come to the second conclusion. (2) Very simple 

 molecules give us coloured light. When this coloured 

 light is analysed by the prism we find associated with the 

 sensation of colour the fact that in this case the light is 

 not continuous ; we do not get a complete spectrum repre- 

 sented by 



^ a © © Y © [^ 



but when we deal with the molecules of one chemical 

 element we may get only 



Y 



in the case of another chemical element only 

 in another 



and so on, the letters representing that light is only 

 given out in those parts of the spectrum represented by 

 them, and not generally. 



This we may also indicate by using black letters for the 

 regions in which light is not given out, and white letters 

 for those where light is emitted, thus 



VI BGVOR 

 VI BGYOI^ 

 W I BGYO m. 



We get bars of light here and there (the various mixtures 

 of which produce different colours), instead of a complete 

 series of bright bars (the mixture of which produces what 

 we call white light). 



The decomposition and recomposition of white light to 

 which I have referred is really one of the most beautiful 

 and at the same time most simple experiments in the 

 whole range of optical science. The recomposition has 



' Continued from p. 6i. 



lately been demonstrated by an elegant toy in the shape 

 of a top, on which, while rotating with considerable 

 rapidity, a circular disc of cardboard containing the dif- 

 ferent colours in their proper proportions painted in sectors 



Fig. 1. — Rotating coloured disc experiment. 



is placed. A lecture-room experiment of the same form is 

 represented in the accompanying woodcut. The various- 

 colours shown on the disc at rest to the right form white 

 light when the disc is rapidly rotated by the handle shown 

 in the figure. 



Two common lustres, or still better, two prisms(Fig. 2), 

 enable the recomposition to be well seen. First arrange 

 one prism as on the right in the accompanying diagram 

 (Fig- 3)' If the eye be placed where the second .prism 

 is to the left, to receive the light passing through the 

 first prism, all the colours will be seen, but if the eye 



Fig. 2. — Prism mounted on stand. 



is replaced by a second prism as shown, the light on 

 emerging from the second prism will be found to be 

 reconstituted, the colours will have again commingled, 

 to form white light. The prism, with its refracting edge 



