Besides cases of complete blindness due to the absence of one or two sensations on the Young theory, we have other cases, as was said when remarking on the percentage of people who are colour deficient, in which one or even two sensations are only more or less deadened. It has often been said that with the theory provisionally adopted, such cases are difficult to class as red or green deficient. As far as my own observations go, I have never found this difficulty. The luminosity curves of such observers, combined with other indications, give a ready means of classing them. The main difficulty to my mind is to state what is normal colour vision, but, as I have found that the very large majority of eyes give the same luminosity to colours as my own, I have taken my own colour perception as normal. In numerous experiments which Lord Rayleigh has made in matching orange by means of a mixture of red and green, he has come across several who have apparently normal vision, as they see colours correctly in every part of the spectrum, and yet some require much less red mixed with the green to make a match with the orange than do others. What is yellow to them is decidedly green to the majority. This has been classed as another kind of normal vision; but the luminosity curves show that it may be equally well due to a deficiency in the green sensation, and which would require more green to make the necessary match. The limits of the visible spectrum to these persons, as far as my examination of their cases goes, are the same as my own.