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a yellow ray, incapable of prismatic analysis into green and red, may 

 be shown to exist, both in the spectrum and in flames in which soda 

 is present ; and 2ndly, that neither red nor green, as sensations, are 

 in the remotest degree suggested by that yellow in its action on the 

 eye. Whether under these circumstances the vision of normal- eyed 

 persons should be termed trichromic or tetrachromic, seems an open 

 question. 



That Mr. Pole's vision is eftchromic, however, there can be no 

 doubt. If I could ever have entertained any as to the correctness 

 of the views I have embodied of the subject in that epithet, after 

 reading all I have been able to meet with respecting it, this paper 

 would have dispelled it. That he sees blue as we do, there is no 

 ground for doubting ; and I think it extremely likely that his sensa- 

 tion of whiteness is the same as ours. Whether his sensation of 

 yellow corresponds to ours of yellow, or of green, it is impossible to 

 decide, though the former seems to me most likely. 



One of the most remarkable of the features of this case, and in- 

 deed of all similar ones, is the feebleness of the efficacy of the red 

 rays of the spectrum in point of illuminating power, which certainly 

 very strongly suggests an explanation drawn from the theory of three 

 primary coloured species of light, to one of which the colour-blind 

 may be supposed absolutely insensible. Mr. Pole himself evidently 

 leans to this opinion. I had satisfied myself, however, in the case 

 of the late Mr. Troughton, that the extreme red that pure and de- 

 finite red which is seen in the solar spectrum only when the more 

 luminous red is suppressed, and in which I cannot persuade myself 

 that any yellow exists, was not invisible to him, though of course not 

 seen as red ; and on supplying Mr. Pole with a specimen of a glass, 

 so compounded of a cobalt-blue and a red glass as to transmit posi- 

 tively no vestige of any other ray, but that copiously, so that a 

 candle seen through it appears considerably luminous, and the 

 window-bars against a cloudy sky are well seen if other light be kept 

 from falling on the eye, I am informed by him that he saw through 

 it " gas, fire and other strong lights with perfect distinctness," and 

 that the colour so seen is a " very deep dark yellow." Now it seems 

 to me impossible to attribute this to any minute per-centage of yellow 

 light of the same refrangibility, which this can be supposed to con- 

 tain. The purity of its tint is extraordinary ; and its total intensity 



