When making matches to simple or other colours by the method of mixtures, we have to be careful of the conditions under which we experiment. This can be shown by a very simple experiment. I will make a match on B with the white light, which is thrown on the surface A (Fig. 6), by mixing the red, green, and violet that pass through the three adjustable apertures or slits already described. The apertures are altered till the match appears to myself perfect. From an appeal made to those of the audience who are at least 25 feet away from the patches of light, as to the correctness of the match, I gather that the match is to them imperfect. The mixed colours appear to them to give a pinkish white. The reason of this defect in the match is due to the fact that, as the lecturer is viewing the two square patches of 2 in. side from a distance of 2 ft. 6 in., their images on his retina extend beyond the boundary of the yellow spot, whilst the audience receives the whole of the image on that portion of the retina which is completely covered by it. To the lecturer only part of the blue and green is absorbed by the yellow spot, and the part of the retina outside it on which the image falls receives and records the full intensity of these colours. To the audience the full amount of absorption takes place, with the result that the patch of mixed colours must appear too red when it is correct to the lecturer. In this case habit makes the eye take an average of the different intensities which must exist at the various parts of the image. We can, however, cause a perfect agreement between all parties if the experimenter views the surfaces in a mirror placed some 12 feet away and then makes the match, for he is viewing the patches from what is practically a distance of 24 feet. If after making the match without the aid of the mirror the lecturer’s eyes are directed a little to one side of the illuminated surfaces, a match will no longer exist; the mixed colour, which is to the audience pinkish, will now appear a bluish green to him. The reason for this alteration in hue is that the whole of the images falls outside the yellow spot. It will now be quite apparent that we must discount any assertion in regard to colour matches, unless we are told the distance of the eye from the surface on which the match is made, together with the size of that surface. This yellow spot is often provokingly tiresome in the study of colour mixtures, and one might almost be justified in doubting whether any _absolutely_ exact matches can ever be vouched for, owing to the important region of the retina which it occupies.