ASTRONOMY, ETC, OF SEVENTEENTH CENT. 219 



so on. Thus he demonstrated that the true cause of the lengthening 

 of the image was no other than that light consists of rays of different 

 refrangibility, which, though incident on the prism at one angle, were 

 refracted in various degrees, and transmitted to different places on the 

 wall. To the same degree of refrangibility belongs the same colour, 

 and each colour has its own degree of refrangibility. Thus, the red 

 rays are always the least refrangible, the violet the most refrangible ; 

 the yellow are more refrangible than the red, and less refrangible than 

 the green rays ; and so on. 



Many experiments with different refracting substances were required 

 to establish the invariability of this law ; for obviously until trial had 

 proved the contrary, there might possibly be some substances which 

 would refract the red rays more than the violet, or the green less than 

 the yellow. And the various coloured rays are plainly contained in 

 the ordinary sunlight, which is thus a mixture of all those coloured 

 rays that are separated by the prism by reason of their different re- 

 frangibilities, and dispersed in more or less diverse directions. 



The colourless or white light of the sun, which was thus separated 

 into rays of the various degrees of refrangibility, Newton conceived to 

 be constituted of the seven different colours already named, viz., red, 

 orange, yellow, green, blue, indigo, and violet: and he measured the 

 relative lengths which these colours appeared to occupy in the spec- 

 trum. We may here remark that although it is quite easy to recognize 

 these colours in the solar spectrum, there are innumerable other tints 

 which would certainly have been reckoned among the constituents 

 had these tints but had a name in common use. It is possible that 

 the mystical significance which we have seen attached to certain 

 numbers, of which seven is one, may have unconsciously influenced 

 the enumeration of the constituents of the solar spectrum. The spec- 

 trum is, however, perfectly continuous, the tints passing from one to 

 another by imperceptible gradations ; and every part is, scientifically, 

 equally entitled to be considered to possess a distinctive colour as 

 those to which the above names have been assigned. 



It should be observed also that the tints of the spectrum are never 

 mixtures of other colours. Thus, for example, though the green lies 

 between the blue and the yellow, it is not a compound of these no 

 further refraction will break up a ray of green light into blue and 

 yellow. There is a very common notion, much fostered by popular 

 teaching, that there are three primary colours, namely, red, blue, and 

 yellow, and that all other colours result from various mixtures of these. 

 This is quite erroneous as regards colours purely, but is related to cer- 

 tain well-known facts in connection with mixtures of 'pigments, and light 

 transmitted through transparent coloured media. 



Newton, having thus analysed light, proceeded to arrange experi- 

 ments for the opposite or synthetical process of recombining the 

 coloured rays. Thus, when all the colours of, the spectrum were 



