110 Dr. W. Pole on some unpublished 



I may now describe the appearance of the colours on each 

 side, beginning with the left-hand or yellow division. Pro- 

 ceeding from the neutral point towards b, the sensation of the 

 yellow colour immediately comes in, but at first very faint 

 and pale, being much diluted with white ; but as we go 

 further the saturation gradually increases until, beyond E, 

 the colour becomes a beautiful resplendent fully-saturated 

 yellow. 



This continues for some distance, until, somewhat beyond 

 D, the strength of the colour begins to fall off again. But 

 the change now is of a nature different from that already 

 described in coming from the neutral point. There is here 

 no dilution with white, the saturation remains full and con- 

 stant, the change being a diminution of luminosity only. 

 This diminution increases 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 *. This fact is very important, for the high 

 saturation of the dark yellow at the end of the spectrum 

 gives so novel an effect as often to lead dichromic patients to 

 believe they really see a third colour there, and so to give an 

 erroneous account of their vision. This error was fallen into 

 by no less a person than John Dalton, and it was the cause 

 of the long delay in the general acceptance of Herschel's 

 dichromic explanation. 



The length of the spectrum visible is, I believe, variable 

 even with normal-eyed persons; with dichromic vision it is 

 often considerably shortened at the red end. But that is not 

 my case ; Sir John Herschel, who had expected a shortening, 

 soon found I had none, and repeated comparisons have con- 

 firmed his statement. 



The arrangement of the colouring in the more refrangible 

 division of the spectrum is precisely similar to that already 

 described. Starting from the neutral point, the blue colour 

 enters, at first much diluted with white, but gradually 

 strengthening till it becomes brilliant and fully saturated 

 between F and G. Beyond this it is gradually darkened, but 

 still preserving its saturation till it disappears. 



The actual vividness, to me, of my two colours, in their 

 maximum saturation and luminosity, i. e. near D and between 

 F and Gr, has been found to be nearly the same as for normal 

 eyes. 



* See some slight indication of this by the increased figure for the 

 saturation of carmine on page 108. 



