There is one curious phenomenon to which attention may be called in this matching, which is worthy of remark. The match will be found to differ according as the patches are compared from a distance of a couple of feet, or from a considerable distance. More green will be required in the latter case than in the former. If matched at a distance of about six feet, and the eyes be then turned so that the edge of the patch falls on their centres, it will be noticed that the colour mixture appears of a green hue. This last experiment indicates that the retina is not equally sensitive for all colours throughout its area. Physiologists tell us that what is known as the yellow spot occupies a central position in the retina, and that it absorbs a part of the spectrum lying in the green. Now when the eyes are close to the patch, its image occupies a considerable part of the retina, and the colour is compounded as it were of the colour as seen on the yellow spot, and of that beyond it, for the yellow spot will take in an image of from six to eight degrees in angular measurement. When viewed at a distance we have the image of the patch falling almost entirely on the yellow spot, and hence a greater quantity of green is required, as it has to make up the deficiency caused by the absorption. When the eyes are turned a little on one side the image falls on the outside of the yellow spot, and the patch illuminated by the mixed light appears green, compared with the patch illuminated with the white reflected beam. It is thus evident that when colour matches have to be made, the distance of the eye from the screen should always be stated, as also the dimensions of the patches viewed. It may be fairly asked why, if the half patch illuminated by the mixed colours appears greener when the eye is turned, the other should not equally do so. This is a very fair question to ask. It must be remembered that one strip is illuminated with white light, in which every coloured ray of light is compounded, whilst in the other only three rays are blended. The green ray chosen happens to be taken from that part of the spectrum which is absorbed by the yellow spot; but all of the green rays of the spectrum are not so much absorbed, hence in ordinary white light, in which all the green rays are present, only a small percentage of the total green in the spectrum is absorbed, compared with that absorbed from the single green ray with which the match is made. No doubt both patches are really greener when the eye receives the impression of their images outside the yellow spot, but one is much greener than the other, and it is thus _comparatively_ green. It is possible to make a match with some colours with a blue-green in which the phenomenon described does not appear; but in cases where a match has to be made with colours in which but little blue is required, it would be impossible to make it, owing to the blue existent in such a green-blue ray.