Data on Colour-Blindness. 109 



Then, finally, there was another incompleteness in my 

 paper, pointed out by Sir John Herschel. He was pleased to 

 say, in a letter to me dated 14th November, 1856, that he 

 " thought my paper had exhausted the part of the subject 

 referring to absorptive or negative colour (that with which 

 painters and dyers were conversant)," but he recommended 

 that I should follow up the communication with another, 

 giving more attention to the prismatic phenomena, which he 

 termed positive colour. Soon after this, Professor Clerk 

 Maxwell undertook to make with me the necessary experi- 

 ments on the prismatic colours, by the aid of the colour-box 

 described in his paper of 1860. They made some progress, 

 but were unfortunately never completed. Since that time 

 several observations on dichromic vision with spectral colours 

 have been published. They agree in general character, but 

 fully confirm the tendency to variability in the quantitative 

 details. It will suffice here to give a general description of 

 the appearance of the spectrum, according to my vision, with 

 the view of showing its analogy to my colour-impressions 

 from pigments, as above described. 



The two diagrams, figs. 3 and 4, will illustrate the com- 

 parison between the normal and the dichromic views of the 

 spectrum. Fig. 3 gives the ordinary Newtonian colour 

 divisions. Fig. 4 describes the appearance of the correspond- 

 ing parts to my eyes. 



My whole spectrum is coloured, mostly very vividly. It is 

 divided into two lengths ; the left-hand division being occu- 

 pied by various modifications of my less refrangible colour, 

 yellow; the right-hand by precisely similar and symmetrically- 

 arranged modifications of my more refrangible colour, blue. 



Much interest attaches to the point of division between the 

 two colours. It must not be supposed that there is any 

 distinct line*, it is simply the place (by no means well 

 defined, for physiological reasons often explained) where the 

 two colours, becoming gradually fainter or paler as they 

 approach on either side, at last lose their hue and merge 

 together. This place is called the neutral point. Its position 

 varies a little with different persons, and even for the same 

 person with different lights ; but it is generally somewhere 

 near midway between the Fraunhofer lines b and F ; and for 

 me its wave-length is about 500. In the normal spectrum 

 this point is, I believe, a powerful blue-green. 



* It is sometimes asserted that there is a colourless band here of 

 greater or less width. I can only say I never saw such a band myself, nor 

 can I understand how such a band could be formed, according to the 

 known origin of the appearances. 



