ISAAC NEWTON loi 



experiment in a well considered manner, and conclusion to 

 conclusion - as we may read in the first book of his Opticks - 

 he arrives at the proof of the compound nature of white light, 

 and the existence of certain mono-chromatic lights which 

 cannot be further decomposed, as they appear in the rainbow 

 or in the prism, passing from red through yellow, green, and 

 blue, to violet. These when mixed give white, and are con- 

 tained, already mixed together, in the original white light. 

 When refraction occurs, as in the raindrop or the glass, the 

 various colours separate, forming the spectrum, and they 

 separate on account of their own different degrees of refrang- 

 ibility. 'Light rays of different colour are also of different 

 refrangibility' is one of the principal results of experiment 

 in Newton's Opticks. 



Every such sentence is always followed by the 'proof by 

 experiment,' mostly by whole series of experiments varied in 

 all sorts of ways, such as to-day also form the introduction to 

 scientific optics. An important matter is the proof that ordi- 

 nary coloured bodies are in no way capable of changing the 

 colour of light rays, but that they merely act selectively, so 

 that red paper for example, simply has the property of re- 

 flecting more red light than light of other colours which falls 

 upon it simultaneously, but not of making, say, red light out 

 of green. If the light of the sun, therefore, were mono- 

 chromatic, say red, as found in the spectrum, no other colour 

 would be visible; all bodies would then only appear more or 

 less bright red, down to black. Only one hundred and fifty 

 years later did Stokes find, in 'fluorescent' and 'phosphores- 

 cent' bodies, cases outside the range of Newton's law; 

 these bodies are actually able to change the colour of light.^ 

 Newton then also showed that all sensations of colour whatso- 

 ever experienced by the eye, may be produced simply by mix- 

 tures of the pure colours from red to violet occurring in the 



1 Stokes lived between 1819 and 1903, and was professor of physics in 

 Cambridge. 



