ISAAC NEWTON 103 



the first place is interested in the hving man, will generally 

 think of colour in the second sense.^ 



The painter will generally take the third meaning as a 

 basis. When he mixes yellow paint (for example gamboge) 

 with blue (for example prussian blue) he gets green. But 

 yellow and blue light when mixed give a more or less pure 

 white. The white light thus obtained is identical for the 

 colour-sensitive eye, and therefore in the second meaning of 

 the word 'colour' above, but by no means in the first meaning 

 of it, with the white of daylight, the latter being a mixture of 

 all colours (in the first sense of the word). These examples 

 serve to show - we have not space here for further discussion 

 - into what confusion we may fall with the word 'colour' if 

 we do not start, as did Newton in his Opticks, from a clearly 

 stated and sharply worded definition. 



The questions of colour have all of them been set upon 

 a firm foundation by Newton's experiments. This was done 

 by observations in experiments which, in part, were not 

 entirely new, but which were used by Newton to draw new 

 conclusions very general in range, and these led him to im- 

 portant developments of knowledge. ^ From what we have 

 already said, the reader will have formed some conception 



^ We note when reading Goethe's theory of colour, that he does not 

 succeed in grasping what Newton meant by colour. Goethe cannot get 

 away from colour impression, and hence does not realise that in order to 

 understand the matters in question as far as possible, it is first necessary 

 to investigate that which produces the impression, that is the light itself, 

 that which passes through space in the form of a ray and possesses pro- 

 perties which are quite independent of the presence of an observer's eye. 

 This is what Newton was investigating. Scientific research must always 

 proceed from the simpler to the more complicated, and not conversely. 

 It is clear from much that Goethe says in his theory of colour, that we are 

 wrong in regarding him as a scientific investigator; he was a great 

 friend and spectator of nature, and that is much, when after the manner 

 of Goethe. 



2 Markus Marci in Prague described in about 1648 experiments with 

 prisms and lenses which in part were similar to those of Newton, without 

 however arriving at clearly defined and regular results, upon which fur- 

 ther construction could take place; also, he appears to be ignorant of the 

 important law of refraction due to Snell (see E. Mach, The Principles of 

 Physical Optics, trans, by J. S. Anderson and A. F. A. Young, 1926). 



