The former were derived from a case P. sent for examination. P. and Q. are brothers, each of whom possesses but one colour sensation, and examination showed that their vision was identical. Mr. Nettleship has kindly given me the following particulars regarding them:--“Their acutes of vision (form vision) in ordinary daylight is only one-tenth of the normal. A younger sister and brother are idiotic and almost totally blind, and in one of these the optic nerves show clear evidence of disease. Hence, the colour blindness of P. and Q. must almost without doubt be considered as the result of disease, perhaps ante-natal, involving some portion of the visual apparatus.” A lack of acuteness of vision would be expected from the small amount of light they perceive compared with normal vision. The fact that two of a family, not twins, possess exactly the same colour sense, and that their extinction curves are entirely different to those suffering from post-natal disease, but similar to those of normal vision, point to their colour blindness as falling in the same general category as that of the congenital type. To this I shall refer again. [Illustration: FIG. 32.] We may reason still further. With the red- and green-blind the violet sensation is still present, and we may therefore expect that their extinction curves, and consequently their persistency curves, should be alike, and should also agree with that made from your lecturer’s observations. A study of Figures 32 and 33 will tell you that such is practically the case. The former shows the luminosity, the persistency, and the extinction curves of a completely red-blind subject, and the latter the same curves for a green-blind subject (see pages 223 and 224). Both were excellent observers, and their examination was easy, owing to the acquaintance with scientific methods. The accuracy of their results may be taken as unquestionable. Each of them may be taken as a representative of their own particular type of colour blindness. There is an agreement between them at the violet end, but a deviation at the red end of the spectrum. The general form of the curves indicates that the same sensation is extinguished last in all. Now, have we any other criterion to offer? We have. In the first instance, we have the violet-blind person to compare with the others, and also another observer who had monochromatic vision, but whose sensation was different to that of the two monochromatic cases we have so far brought to your notice. We have already stated the peculiarities in colour nomenclature of the violet-blind case. His curve of luminosity for the spectrum was taken (page 227), and when compared with the curve of normal luminosity, it became evident that in the red and up to the orange his measures were those which a normal eye would make; but that the luminosity fell off in the green, and finally disappeared to an immeasurable quantity in the violet (see Fig. 30, curves M and F).