The next test was made to throw light on this point. He made observations of the extinction of the different parts of the spectrum. His observations were very fair, except on the violet side of F, where they became slightly erratic, but by requesting him to use all parts of his retina to obtain the last glimpse of light, a very concordant curve resulted, as shown in Fig. 34. Some of his observations at this part were evidently made with the centre of the retina, for they gave readings which, when the “persistency” curve was calculated, and these observations treated as part of the extinction, agreed with the luminosity curve. We may, therefore, conclude that B. C. has a region in the retina in which there is an absorbing medium corresponding to the yellow spot of the normal eyed. This is diagrammatically shown in Fig. 34 by the difference in height of ordinates in the persistency (dotted) and the luminosity curves. On the red side of the maximum the two curves are practically identical, except from Scale number 54. At this point it is probable that the white light which illuminated the prism vitiated the readings to some degree. At the violet end something similar, doubtless, occurs, but it is masked by the difference that exists in the extinction by the central part of the retina and that of the whole eye. It must, however, be remarked that the amount of reduction of the intensity of a ray to produce extinction is very different for B. C. and for the normal eyed, or for the red- and green-blind or for P. and Q. B. C. can bear nearly 200 times less reduction for the rays near E. We have already pointed out that the same is practically the case with M., whom we presume to be violet-blind. We may therefore deduce the fact that the monochromatic vision in this case is of a totally different type to that of P. and Q., and that the last sensation to be lost is the same as that of M. If any violet sensation were present in either, the fact would be made evident by the order of the extinction. The sensation of B. C. is thus apparently the green sensation, though that this particular sensation is exactly the same as that absent in the green-blind is not certain. The observations made by the different types of the colour blind seem to me to throw great light on the theory of colour vision. They show that when the violet sensation is present, according to the Young theory, the extinction shows its presence; and that where this sensation is absent, the reduction of light necessary to produce extinction is greatly less, and may with great certainty be attributed to a different sensation being the final one to disappear. CHAPTER XI.