It may be further remarked, that if the maxima of all the three colour sensations are taken, as in the diagram, as of equal value, that the place in the spectrum where the white light is perceived by the colour-blind is where the two sensations are of equal strength, that is, where the two curves cut one another, and are of equal height. By obtaining the proportions of the different colours with colour-blind persons which make up what to them is white light, the curves for the two sensations can be worked out in the form of simple equations. The experiments carried out with colour-blind people are of the most interesting character, and a good deal remains to be done with the data already obtained from them. To the popular mind a colour-blind person is usually thought a strange creature, and it is a matter of wonderment, if not of amusement, that they cannot distinguish between the red of cherries and the leaves of the cherry tree. The physicist, studying the theory of colour, views the matter quite differently, and he looks upon an intelligent observer of this class as a boon. It may be remarked that both the red-blind and the green-blind persons would be unable to distinguish between the cherries and the leaves. The red-blind person would see the cherries as green, as also the leaves; whilst the green-blind person would see both as red. Without regarding form it is probable that the red-blind would see the leaves as a bright green, whilst the green-blind would see them as darker red than the cherries. Failure to distinguish between the two is more likely to occur with the green of leaves, and the red of such fruits as cherries, since the former contains a marked proportion of red in it, and the latter a small proportion of green. One highly-educated gentleman was led to know his deficiency in colour sense, by hearing a companion on a tour going into raptures over a sunset. He saw but little difference between it and that to be seen at midday. Testing his vision it appeared that he was totally blind to the sensation of green, and that white and purple would consequently be mistaken by him for one another. The crimson on the clouds, illuminated by the setting sun, would appear to him as only slightly different to the white clouds which he would see at midday; in fact he would be always seeing what to us would be a sunset. For this gentleman to mix spectrum colours to match others would evidently be no guide to normal-eyed persons. We believe that amongst us in our daily life we have many persons who are blind to some colour, but who are not aware of it, or if they are aware of it, hide their defect as far as possible. That some are ignorant of it to a late period of their life we know.