one of monochromatic vision, is eagerly sought after, with the hope of submitting it to a rigid examination. When we look at the diagram (Fig. 16) we shall find why it is that the colour blind describe the spectrum as they do. Literally for those whose vision is dichromic, it is made up of two sensations alone, and the colours to which these sensations give rise are mixed throughout a large part of the spectrum, the pure unmixed sensations being at each end of the spectrum as they are in normal colour vision. The annexed diagram (Fig. 18) gives the curves for a red-blind person as made by observations under Clerk Maxwell’s directions. The standard colours here have been badly selected, for one of them stimulates the two sensations possessed. [Illustration: FIG. 18.] An easy and instructive experiment can be made to give an idea of the kind of colour that these colour blind imagine as white, whether they be red-, green-, or violet-blind. (For those who have only monochromatic vision, as before stated, white is coloured with the one colour they possess.) Three slits are now in the spectrum, one near the extreme end of the red, another well in the violet, and the third in that part of the spectrum in which the green-blind see their neutral colour (see page 66). With the three colours issuing from these apertures a match is made with the white patch, and in this case the match is made as seen from a distant point, so that the resulting deductions may be true to the audience. If a colour-blind person be in this theatre, he will agree with me that the match is as correct to him as it is to myself and the rest of you. So far we could not distinguish his colour perception from the normal, but if he be red-blind, and the red slit be covered, he will still say that the match holds good, for, as a matter of fact, the red with which we helped to build up the white is non-existent to him. The white that he now sees is to us the greenish-blue patch which the mixed violet and green make. If he be a green-blind person he will tell us the colour is a very pale blue, but when the green slit is covered up and the red uncovered, the match will once more be correct, though the purple, formed by the mixture of red and blue, will appear to him to be a little darker than the white. This is what one would expect, for you must recollect this green in the spectrum he would call white or grey. If then, from what to him is also white, though formed by the rays coming through the three slits, we take away a certain amount of degraded white (green to us), he must still see white, but darker. We have, however, met with what is an apparent paradox. The green, coming through the now covered slit, he calls white, as he also does the purple. To impress this point more strongly upon you, I will place in front of the green slit a small prism which has an angle of about one and a-half degrees. This is just sufficient to throw the green colour on the neighbouring white surface.