Clifton College Scientific Society. 83 



hole in a shutter into a dark room, and then examined it after it 

 had passed through a triangular piece of glass called a prism. He 

 found it was decomposed into lights of various colours, passing 

 from red through all the shades of orange, yellow, green, blue, and 

 violet. He also found that these rays could be again resolved into 

 white light, and this he showed by allowing the coloured band to 

 pass through an inverted prism. If, however, we cut off a portion 

 of the spectrum which has passed through the first prism, on 

 emerging from the second it will have undergone no alteration. 

 The green rays, for example, will still appear green. But not 

 only does the mixture of all the coloured rays give white light, 

 but different other mixtures are found to produce the same result. 

 Thus, violet and greenish yellow, indigo and yellow, blue and 

 orange, greenish blue and red, and blue, orange, and red, give, 

 wlien mixed, white light. It was once believed that the colours 

 of the spectrum overlapped each other, — that is to say, it was com- 

 posed of three principal colours, red, yellow, and blue, and that 

 orange, green, and violet were formed by the overlapping of red 

 and yellow, yellow and blue, blue and red respectively. But this 

 has been found not to be the case, for the green, or orange, or 

 violet, cannot be decomposed into any other colours. 



Now, why is it that the eye can distinguish different colours ? 

 As light is a wave motion, we conclude that different colours are 

 caused by different undulations of the elastic medium pervading 

 all space. And here we may compare sound to light. As sounds 

 of different pitch and intensity are caused by different rates and 

 amplitudes of vibrations respectively, so are different colours and 

 shades produced. But we know that sound travels at the rate 

 of 1090 feet per second in air (at 0° C), and is not transmitted at 

 all in vacuo, whilst light travels at the enormous velocity of 

 190,000 miles a second. The wave length of the red ray A* just 

 visible to the eye is ~^„o of ^.n inch, and in one second no 

 less than 458,000,000 O'OO.OOO vibrations occur. The wave length 

 of the violet ray H is j^^^, of an inch, and 727,000,000,000.000 

 vibrations occur in one second. Hence the difference is only from 

 458 to 727, or the rapidity of vibration at the one point is not 

 twice as great as it is at the other, so that, although the human 

 ear is capable of hearing about eleven octaves, the eye cannot 

 see even one. Nevertheless, there exist rays beyond the red and 

 violet which we call obscure. It seems a strange fact, but still it 

 is true, that half the rays of the sun reaching the earth are invi- 

 sible. If we pass the rays coming from the electric light or from 



* From the cliflBculty of procuring coloured illustnations of the spectra, it 

 has been necessary to omit them ; but the letters here given refer to the excellent 

 plate in Mr Roscoe's ' Elementary Chemistry,' a work readily accessible to our 

 readers. — Ed. 



