258 SCIENCE PROGRESS 



be in proportion to one another, as the numbers i , f , f , f , f , f , A, 

 £, and so to represent the Chords of the Key, and of a Tone, a 

 third Minor, a fourth, a fifth, a sixth Major, a seventh, and an 

 eighth above that Key. And the intervals Ma, ay, ye, erj, r\i t 

 i\, and XG, will be the spaces which the several Colours (red, 

 orange, yellow, green, blue, indico, violet) take up." 



This account in the Opticks was, of course, published in 

 1704, thirty-eight years after the experiments were made, and 

 eight years after Newton became Warden of the Mint, and 

 represents his final view of the matter. The parallel passage 

 in the Lectiones Opticce, or Optical Lectures, which according 

 to their preface would have been published by Newton about 

 1 67 1, had it not been for his dislike of controversy, is much 

 fuller, and shows us his views in course of formation. 



The passage begins on p. 239 x and describes how the spec- 

 trum was received on a piece of paper, and the boundaries of 

 the different colours as well as their regions of greatest purity 

 marked on the paper, the " ordinary camera " arrangement 

 being used. Then on p. 240 the results of this investigation 

 are given under five heads : 



(1) The boundary line between blue and green divides 

 the length of the spectrum into two equal parts, blue 

 (caeruleus) and violet (violaceus) composing the one part 

 and green (viridis) and red (rubeus) the other. The colour 

 between blue and green is referred to as thalassinus — sea- 

 green . 



(2) A line drawn across the spectrum at the richest part 

 of the green divides the spectrum in the ratio 3 to 5 . 



(3) The length of the green is one-sixth the length of the 

 spectrum. 



(4) The boundary between blue and violet (this time 

 purpureus is used for violet) or the most perfect indigo 

 (indicus) is T V of the length of the spectrum distant from 



1 The reference here is to the Latin edition published in 1729, two years after 

 Newton's death. The lectures were delivered at Cambridge in 1669, and the 

 Latin original was deposited at the time it was read in the archives of the univer- 

 sity. The lectures are divided into two parts. The second part, from which the 

 matter here cited is taken, relates to the doctrine of colours, and was recast and 

 improved by Newton in the Opticks. The first part is preparatory to the second, 

 and contains much less in common with the Opticks. It was published by itself 

 in an English translation in 1728. 



