NEWTON AND THE COLOURS OF 

 THE SPECTRUM 



By R. A. HOUSTOUN, M.A., Ph.D., D.Sc. 



Lecturer on Physical Optics, University, Glasgow 



Every student of Optics knows that when Sir Isaac Newton 

 discovered the spectrum he divided it into seven colours, 

 namely, red, orange, yellow, green, blue, indigo and violet, 

 and most students of Optics have at the same time wondered 

 why indigo was included in this list of colours. For if we look 

 along the spectrum from the one end to the other, screening off 

 everything except the particular strip we are examining, we see 

 in succession red, orange, yellow, yellow-green, green, green-blue, 

 blue, blue-violet, and violet, but between the blue and the violet 

 there is nothing sufficiently different in kind to warrant the 

 introduction of a new name ; the one colour merges gradually 

 into the other, and the intermediate shades are clearly mix- 

 tures of blue and violet. Indigo, as we know it from the water- 

 colour paintboxes, is an inky blue, a blue inclined to black, 

 not one inclined to violet. Indeed the late Prof. S. P. Thompson 

 gave it as his opinion in his lectures that indigo was inclined 

 more to green than to violet, and in this opinion, I think, every 

 one will concur. What did Newton understand by the colour 

 indigo, and why did he attach such importance to it ? 



There is no doubt, I think, that Newton introduced his 

 seventh colour partly because his own colour vision was slightly 

 abnormal, partly from a fancied analogy between the spectrum 

 and the musical scale, influenced no doubt by Kepler's doctrine 

 of harmonies. Also I think it highly probable that it was on 

 account of this fancied analogy that he regarded the dispersion 

 of all transparent substances as the same, and did not attempt 

 to correct the chromatic aberration of lenses. It is always 

 interesting to consider the mistakes made by eminent men, 

 especially when they are so important for the history of science 

 as this one was. So I propose in this paper to give an account 



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