SCIENTIFIC NEWS. 



[June 1st, 1887. 



ordered to be transmitted to its author for his "very ingeni- 

 ous discourse." 



The refraction of light by glass and crystal had been 

 investigated by Ptolemy in the second century. Lord 

 Bacon described no new discovery when he wrote in his 

 Sylva Sylvarum or Natural History, " Precious stones have 

 in them fine spirits as appeareth by their splendor. Prismes 

 are also comfortable things"; or in his Novum Organum, 

 " If the nature of colour is inquired into, the solitary in- 

 stances are gems of christal, which yield not not only a 

 colour in themselves, but cast it upon a wall."* But it was 

 generally supposed that light of every colour was equally 

 refracted or bent in its path by a lens or prism. 



Newton " let in a convenient quantity of the sun's light " 

 through a hole in the shutter of a darkened room. " It was 

 at first a very pleasing divertissement to view the very 

 vivid colours produced thereby." But he was not so much 

 struck by the colours, nor did he stop to speculate how 

 " they may work by consent upon the Spirits of Men, to 

 comfort and exhilarate them," as did Lord Bacon, but was 

 interested in noticing the length of the band of coloured 



Fig.; I. 



light upon the wall compared with its breadth. The hole 

 was round, but the band was five times longer than its 

 breadth. 



" He thought it not amiss to examine. . . .what would hap- 

 pen by transmitting light through parts of the glass of 

 divers thicknesses, or through holes in the window of 

 divers bignesses. . . .buthe found noneof these circumstances 

 material. The fashion of the colours was in all cases the 

 same." After making various experiments he imagined the 

 rays might take curved paths, for " he had often seen a tennis 

 ball struck with an oblique racket describe such a curve 

 line " ; but " this plausible ground for suspicion " was 

 destroyed by further observation. 



At length he let the coloured band, or spectrum, from the 

 prism fall on a board with a hole in it, and thus allowing 

 one colour to be dealt with at a time, he passed this colour 

 through a second prism, and allowed the light to fall on 

 another screen. The ray examined in this manner apart 

 from the rest, underwent no further change beyond a 

 slight elongation. 



Turning the first prism so that one colour after another 



*" Translated and taken out of the Latine by M.D, B.D. 1677." 

 The second not is apparently a misprint. 



passed through the hole and fell on the second prism, he 

 noticed that the rays were bent at different angles. He was 

 then led to make his great discovery, which may be stated 

 thus ; white light consists of rays differing in colour and 

 refrangibility. He described the colours as seven in num- 

 ber — violet, indigo, blue, green, yellow, orange, and red. But 

 it was explained in our Notes on Colour* that indigo should 



Fig. 2. 



not rank as one of these. The red is the least bent and 

 the violet is bent the most. 



Before proceeding to discuss the spectrum more minutely, 

 it will be well to describe by an analogy how the light is 

 acted on by the prism. 



When a beam of light passes from air into water or glass, 

 meeting the surface at an angle, it becomes bent. This 

 happens because light travels more slowly in the denser 

 substance. A similar deviation would take place in the 

 direction of march of a column of soldiers moving blindly 

 or in the dark, if they came upon a field of heavy soil or 

 rough ground. 



The man at A on the left-hand side of the column would 

 be the first to meet with the retardation, and while he 

 marched from A to C the corresponding men at B would 

 have marched to D. The effect would be to swing the 

 front of the column round, and so change its direction. If 

 the further boundary of the heavy ground were parallel to 

 that at which it was encountered, the column would resume 

 its original direction of march) but if not, another deviation 

 would take place, as in fig. 2. A ray of one colour only 



* Vide ante p. 12, 



