NEWTON AND THE COLOURS OF THE SPECTRUM 259 



the boundary between red and yellow (flavus) or the most 

 perfect orange (citrinus). 



(5) The distance from indigo to orange is divided by 

 the boundary of the green and blue in the ratio 2 to 3, the 

 line of division being nearer the indigo. 



He states that he arrived at these results not so much 

 trusting to his own perception, but relying on the judgments 

 of others (aliorum judiciis fretus) . He next exhibits the results 

 in a diagram, fig. 38 of the Lectiones Opticce ; the figure above 

 is a somewhat simplified form of this diagam. The diagram 

 shows the whole spectrum divided into sixty parts, and the 

 numbers represent the distances of the points of separation of 

 the colours from the red end. The diagram gives only the five 

 " insigniores colores," as they are referred to. So far orange 

 and indigo have no more claim to importance than sea-green has. 



We come now to the reason for their addition to the other 

 five. The purest green lies midway between H and I in the 

 position shown by the dotted line (in this passage in the original 

 all the letters in the text have accidentally been shifted one 

 place to the left, but it is easy to make the necessary correc- 

 tion), but the purest violet, blue, yellow, and red do not occur 

 in the middles of their respective spaces. The purest blue and 

 yellow are nearer the green, and the purest violet and red 

 nearer the ends of the spectrum in the positions shown by the 

 dotted lines. Since the intervals between the purest blue and 

 violet and between the purest red and yellow are about one- 

 third greater than the intervals between the purest green and 

 blue and purest green and yellow, in order to divide the image 

 more elegantly into parts (quo imago elegantius in partes inter 

 se proportionates distinguatur) it is advantageous to admit two 

 other colours into the number of the five more important colours, 

 namely, orange between red and yellow and indigo between 

 blue and violet. 



Then Newton says he has investigated the limits of the 



