With the partially colour blind the same procedure may be adopted. In examination by the Holmgren wool test, slight mistakes will be made in matching the first two test-skeins. With the spectrum test the red will require a greater dilution with white before it will be matched with a green, even if it can be matched at all. Measures of the luminosity at four or five positions in the spectrum, extending from near the extreme red to the blue, will give an unerring criterion of the kind and extent of colour blindness from which they are suffering. The existence of a neutral point in the spectrum is sufficient to indicate that their blindness is of a nature to be dangerous in certain occupations. To some it may be a difficulty how a neutral point can be found in such cases, since all sensations are more or less present. The reason, however, was explained on page 96. CHAPTER XV. Examples of colour blindness have been brought to your notice, and various measurements made by persons possessing normal and defective colour vision have been recorded, but no attempt has been made to discuss the two leading rival theories that have been laid before you. Regarding these theories you may expect me to say something, and to avow myself a partisan of one or the other. This last I must decline to do, though it will have been seen by the line that I have taken in these lectures that the Young theory attracts me. There are, however, difficulties in adapting it to explain several facts of colour vision which seem to render it, to say the least, incomplete. For instance, to explain the colours produced by simultaneous contrast, the Young theory has to betake itself into psychological ground. I will show you some excellent examples of contrast colours. We have upon the screen a patch of white reflected light, superposed over a patch of red light. Placing a thin rod in the paths of the two beams, we have two shadows--one illuminated by white and the other by red, and lying between them a mixed light of red and white. The shadow illuminated by the white does not appear white, but a bluish-grey. When the spectrum colour is changed to orange the blue is intensified, whilst when it is green, what should be white appears of an orange-salmon colour. Other colours give the white different hues which I need not describe.