104 A CHAPTER ON LIGHT AND COLOUR. 



found to be possible to rotate the wheel so fast that light which, 

 emerging between two of its teeth, passed on to be reflected back 

 from a mirror five miles distant, returned to the wheel to find its 

 path blocked by a tooth. A more rapid rotation permitted it to 

 pass through the next gap. Knowledge of the distance traversed 

 by the light and the speed of rotation of the wheel enabled the 

 speed of light to be calculated. Another method^ due to Foucault, 

 was devised about the same time, which involved the use of a 

 rapidly revolving mirror. The distances used were in this case 

 shorter by far than in Fizeau's method. 



By the application and perfection of these methods, the Velo- 

 city OF Light has been found to be about 186,400 miles per sec. 

 The length of an undulation is extremely small, however, being only 

 about i/4o,oooth of an inch, so that about 500,000,000^000,000 

 of waves are received into the eye in each second. 



7. — Colours. — When light is refracted through a wedge-shaped 

 piece of glass, known as a Prism, it is found that, instead of a 

 single image of the luminous object, a coloured band of greater or 

 less length is obtained. The various colours may be recombined 

 in various ways, and are found to give, as a result, simple white 

 light. The spreading out of light in this way is called Dispersion, 

 and the power of producing it varies in different transparent sub- 

 stances. It affords evidence that white light is not simple, but 

 complex, and that the variously coloured beams which go to make 

 up a beam of white light are refracted by slightly different amounts 

 in the same refracting medium. 



The coloured band, if the dispersion be great enough, is found 

 to contain the colours in the order, red, orange, yellow, green, blue, 

 indigo, violet. The red end is the least refracted, the violet end 

 the most. The coloured band is called a Spectrum. 



8. — The mixing of various colours produces on the eye the effect 

 of some other simple colour tint, but the Spectroscope at once 

 shews that the colour seen is a mixture, not a pure colour. It has 

 been maintained that suitable combinations of vermilion, emerald 

 green, and ultramarine properly chosen can be made to give any 

 desired colour sensation. Others maintain the necessity for five 

 primary colours. The mixture of colours and the mixture of the 



