444 I)R WILLIAM POLE ON THE PRESENT STATE OF 



be worth while for me to examine carefully my own colour-sensations, and to see whether 

 or not they corresponded strictly with the dichromic explanation. I found that they did 

 so, and I wrote a paper " On Colour-Blindness," endeavouring, by the description of my 

 own case, not only to prove this fact, but to account for its previous obscurity. The 

 communication was read before the Royal Society of London on the 19th June 1856, and 

 was referred to Sir John Herschel, whose favourable report on it was (contrary to the 

 usual custom) partly published in the Proceedings, I was recommended to amplify it by 

 some experiments on Maxwell's new system, after which it was printed in the Phil. 

 Trans, for 1859. My verification of the dichromic theory was fully adopted and reasoned 

 on in a second communication, by Clerk Maxwell, to the Royal Society of London in 

 1860, and since that, Herschel's most happy simplification of Colour-Blindness has been 

 universally received. 



There were certain points of my paper left obscure and unfinished, and to clear them 

 up I have published some supplementary remarks in the Philosophical Magazine of 

 July 1892. 



The general features of dichromic vision have been determined by a large number of 

 observations, and may be summed up as follows : — 



The solar spectrum shows, to a dichromic eye, only two hues — a warm and a cold one — 

 separated from each other at a rather uncertain colourless point, called the Neutral Point, 

 lying in the normal blue-green, between Fraunhofer's lines b and F. The warm hue 

 occupies the whole of the less refrangible part to the left of the dividing place ; the cold 

 hue occupies the whole of the more refrangible part to the right hand. These two colours 

 will be complementary with respect to the patient's white. 



The two colours are modified throughout the length of the spectrum by variations in 

 their saturation, or their luminosity, or both. I may quote a description of the appear- 

 ances to me, according to various observations at different times, some notes of which I 

 have given in the " Data," AG. 



My neutral point, or division between the colours, lies at a somewhat uncertain wave-length not 

 far from 500, and appears a dull, colourless hue, not white, but a sort of grey. Starting from this, 

 and proceeding to the left hand towards b, the sensation of the yellow immediately enters, at 

 first very faint and pale, being much diluted with white ; but as I go further the saturation 

 gradually increases until, beyond E, the colour becomes a beautiful, resplendent, fully-saturated 

 yellow. The place of maximum strength is difficult to determine, but it is probably shortly before 

 reaching the line D. Soon after passing D the colour begins to fall off, but the change now is of 

 a nature different from that above described in coming from the neutral point. There is here 

 no dilution with white ; the saturation remaius full and constant, the change being a diminution of 

 luminosity only. This diminution goes on gradually, giving an appearance like that of the shading 

 of a round column in an engraving, and it increases in darkness till the visible spectrum ends. But 

 the saturation of the colour is fully maintained the whole way, and some good authorities contend 

 that it even increases towards the end. 



The disposition of the colour-impressions in the more refrangible division of the spectrum is 

 precisely similar. Starting from the neutral point, and proceeding towards the right hand, the blue 



