366 CELESTIAL CHEMISTRY. 



But refraction does more than merely bend a ray of light ; 

 it sifts it or breaks it tip in the way that I shall now proceed 

 to explain. Alost of us have seen those revolving cylinders 

 that are used for sorting stones for macadamising the roads. 

 They have a series of very small holes at the upper end, larger 

 holes lower down, and largest of all at the very bottom : the 

 cylinder revolves around a sloping axis, and when a miscel- 

 laneous collection of stones is thrown in the very smallest 

 stones fall out first and form a heap near the upper end of the 

 machine, while the largest stones slip down all the way and 

 fall out only when they reach the large holes at the vert^ bottom, 

 so that we get a succession of heaps of stones sorted out 

 according to their various sizes. I want to make clear to you 

 how we can sort out light in a similar manner, and then to 

 read in those little patches of light, so sorted out, the 

 marvellous messages they bring to us from those distant orbs 

 whence they set out it may be thousands of years ago. 



If w^e look through a prism at two narrow strips of coloured 

 paper — one red and one blue — placed absolutely side by side 

 with their edg'es in a straight line, we shall of course see that 

 both strips are displaced, like the end of the stick in the pool 

 of water : but we shall notice one remarkable thing — the dis- 

 placement of the blue strip will be greater than that of the red 

 one. This can mean nothing else than that the rays of blue 

 light undergo greater refraction than rays of red light. Now 

 what follows from this ? If a beam composed only of red and 

 blue light passes through a prism, the prism will split up the 

 light into its two colours and we shall see two spots of light 

 — one red and one blue. On the other hand, if a beam of light, 

 made up we don't know how, passes through a prism, and 

 produces two light spots, one red and one blue, we shall be 

 quite correct in concluding that the original beam was made 

 up of a mixture of red and blue light. 



Now we may take a step further. Sir Isaac Newton, in 1674, 

 allowed a round beam or pencil of sunlight to pass through a 

 hole in the shutter of a darkened room, to traverse a prism, 

 and then fall upon the wall opposite. He found that this 

 entirely changed the character of the light. From a round 

 pencil of white light it had been altered into a long band 

 showing an immense variety of colours. We generally speak 

 of them as seven — violet, indigo, blue, green, yellow, orange, 

 and red — but in reality they shade off into each other in a 

 series of imperceptible gradations. In such a case the violet, 

 which undergoes most refraction, is deflected towards the base 

 of the prism, and the red, least refracted, occupies a position 

 towards the opposite edge or apex of the prism, and the whole 

 band of colours is called a spectrum. 



A prism, by the way, is not of necessity triangular in shape : 

 any transparent material, of which the opposite sides, no matter 

 whether they be flat or curved, are not parallel to each other, 

 will act as a prism. Thus a decanter of water may produce a 

 display of prismatic colours, and even a single drop of water 

 will do so under suitable circumstances : each raindrop, in fact, 

 is a refracting medium, and when the direct sunlight falls upon 



