252 SCIENCE PROGRESS 



its limits. The spectrum was that of a carbon arc with a 

 somewhat sudden increase of brightness in the violet, and I 

 was afraid this increase of brightness might be misleading. 

 So those who said they saw indigo were taken to a spectroscope, 

 shown a very good continuous spectrum, and asked again to 

 mark the limits of indigo ; also two of the four were shown 

 monochromatic patches, and asked to say rapidly whether 

 these were blue, indigo, or violet, an attempt being made to 

 confuse them. 



I had no difficulty in convincing myself, somewhat to my 

 astonishment, that the four students referred to have a much 

 better power of discriminating hues in this part of the spectrum 

 than I have, and my colour vision is normal. They all objected 

 to the word indigo, and chose dark blue as a more suitable name 

 for the new colour ; they all said it was liker blue than violet, 

 and their readings for the boundary between it and blue were, 

 in /A/Jb, 457, 455, 465, 465, 465, the first two readings being by 

 the same observer, and all the readings being made rapidly and 

 without hesitation. At this part of the spectrum I see only 

 blue merging into violet, and I find it extremely difficult to 

 say where the one begins and the other ends, getting anything 

 between 437 and 460 /a/*. 



In his Opticks Newton refers to his assistant in connection 

 with the discrimination of colour, and in the Lectiones Optica 

 speaks of himself as relying on the judgment of others. He 

 would naturally choose others with a keen sense of colour, but 

 he would not, I think, have committed himself so definitely 

 had he not seen indigo to some degree himself. I am there- 

 fore inclined to class him as a doubtful heptachromic and his 

 assistant as a decided heptachromic. 



Before describing Newton's experiments I should like to 

 dismiss a suggested explanation which has been frequently 

 offered to me by students, namely, that indigo was introduced 

 to make out the seven colours, that Newton was anxious to 

 have seven colours in his list on account of some mystical 

 significance attaching to that number. The words of Helm- 

 holtz x are, I think, quite conclusive on this point, namely, 

 that to the observer with normal colour vision orange-yellow, 

 yellow-green, and sea-green, i.e. green-blue, are at least as 

 different from their neighbours in the spectrum as indigo is 

 1 Handbuch der Physiologischen Optik, 2nd edition, p. 288. 



