equally so in lamplight, although some shades of colour which are well distinguishable by daylight may disappear when the artificial light is used as the source of illumination. Now, on what scientific principles should a colour test be founded? We must hark back to a theory for a moment, and as it has been shown that for all essential purposes that of Young answers, we will use it as a good working hypothesis, and it was from this theory that Holmgren himself reasoned. The red- or green-blind see a grey in a part of their spectrum, which to us who possess normal colour vision is green. If then we present such a green to them, they would match it with a grey. If, however, we have a yellowish-green, which is pure green mixed with red, the complete green-blind will not see the green in it, but only the red. The colour to him would be very pale red, and as he sees all such greens and yellows and reds as red more or less saturated, that is, more or less mixed with his neutral colour, any one of these he would match with a green. The red-blind, on the other hand, would see all these colours as green, and he too might make similar matches with them. Suppose now we have a pink skein: the green-blind would see it as a white or bluish-white, for a purple is white to him, and he would match with it either greys or colours having a slight excess of blue in them; for a green is to him a neutral colour. The red-blind, on the other hand, would see but little green in the pink; blue would predominate, so he would choose mauves or blues amongst other matches.