If his measures of spectrum luminosity are deducted from those of the normal eye, and the ordinates be increased proportionately to make the maximum difference 100, the figure so produced, when compared with the _luminosity_ curve obtained from the monochromatic observers, was found to be the same, and consequently with the persistency curves above referred to. Endeavours were made to gain a good extinction curve, but the results were not as successful as could be desired; but it was ascertained that, without doubt, his most persistent sensation was not more than 1/180 as lasting as that of the normal eye, or to put it in another way, his green at E was only extinguished when the energy falling on his eye was 180 times greater than that at which it vanished with the normal eye. This plainly teaches us that the missing sensation was that which, when present, is ordinarily the most persistent. The next is a case of monochromatic vision, which differs from those previously brought before you, and I cannot do better than describe it in the words which General Festing and myself employed in our paper in the “Philosophical Transactions.” [Illustration: FIG. 33.] The patient (B. C.) had been examined by Mr. Nettleship, who kindly secured his attendance at South Kensington for the purpose of being examined by the spectrum and other tests. [Mr. Nettleship states that this case is without doubt a genuine case of congenital colour blindness, without any trace whatever of disease.] B. C. is a youth of 19, who has served as an apprentice at sea. His form vision is perfect, and he is not night blind. He can see well at all times, though he states that on a cloudy day his vision seemed to be slightly more acute than in sunshine. He was first requested to make matches with the Holmgren wools in the usual manner, with the result that he was found to possess monochromatic vision. He matched reds, greens, blues, dark yellows, browns, greys, and purples together; and it was a matter of chance if he selected any proper match for any of the test colours. Finally, when pressed, he admitted that the whole of the heap of wools were “blue” to him, any one only differing from another in brightness. The brighter colours he called “dirty” or “pale” blue, terms which eventually proved to be synonymous. We then examined him with patches of monochromatic spectrum colours by means of the colour patch apparatus. He designated every colour as “blue,” except a bright yellow, which he called white, but when the luminosity of this colour was reduced he pronounced it a good blue. So with white, as the illumination was decreased, he pronounced it to pass first into dirty blue, and then into a full blue.