610 Captain W. de W. Ahney [May 6, 



the reciprocals of the numbers which give extinction of the spectrum, 

 we ought to get the curve of the violet sensation on the Young-Helm- 

 holtz theory. For if one violet sensation has to be reduced to a 

 certain degree before it is unperceived, and another has to be reduced 

 to half that amount, it is evident that the violet sensation must be 

 double in one case to what it is in the other ; that is, the degrees 

 of stimulation are expressed by the reciprocal of the reduction. 



Such a curve is shown in Fig. 5 (in which also are drawn the 

 curves of luminosity of the spectrum when viewed with the centre 

 of the retina and outside the yellow spot). And it will be noticed 

 that it is a mountain which reaches its maximum about E. Remem- 

 ber that the height of the curve signifies the amount of stimulation 

 given to the violet sensatory apparatus by the particular ray indicated 

 in the scale beneath. 



Turning once more to Fig. 3, it will be noticed that if any one 

 or two of the three sensations are absent, the persons so affected are, 

 what is called, colour blind. Thus if the red sensation is absent they 

 are red blind ; if the green, then green blind : if the violet, then 

 violet blind ; if both red and green sensations are absent, then the 

 person would see every colour, including white, as violet. The 

 results of the measurement of the luminosity of the spectrum by 

 persons who have this last kind of monochromatic vision should be 

 that they give a curve exactly, or at all events very approximately, of 

 the same form as the curve given by the reciprocals of the extinction 

 curve obtained by the normal eye, as the violet sensation is that 

 which is last stimulated. 



It has been my good fortune to examine two such persors, and I 

 find that this reasoning is correct, the two coinciding when the curves 

 for the centre of the retina are employed. 



Further, I examined a case of violet blindness, and measured the 

 luminosity of the spectrum as apparent to him. Now if the Young- 

 Helmholtz theory be correct, then in his case the violet sensation ought 

 to be absent, and the difference between his luminosity and that of 

 the normal eye ought to give the same curve as that of the violet 

 eensation. This was found to be the case. 



Again, the reciprocal of the extinction curves of the red blind 

 and green blind ought to be the same as those of the normal eye, 

 for the violet sensation must be present with them also. This was 

 found to be so. We have still one more proof that the last sensation 

 to disappear is the violet. 



If we reduce the intensity of the spectrum till the green and red 

 disappear to a normal eye, and measure the luminosity of the spec- 

 trum in this condition, we shall find that it also coincides with the 

 persistency curve. On the screen we have a brilliant spectrum, but by 

 closing the slit admitting the light and placing the rotating sectors 

 in the spectrum and nearly closing the apertures, we can reduce 

 it in intensity to any degree we like. The whole spectrum is now 

 of one colour and indistinguishable in hue from a faint white patch 



