Mistakes which are due to shortening of the spectrum may be remedied if we subtract the rays occupying the missing portion from the colour of confusion. For instance, if we take a blue and a pink which have been put together as identical by a person with a shortened red end of the spectrum, and look at them through a glass which is opaque to the red, but transparent to the remaining rays of the spectrum, both will appear alike in hue and shade. A person with considerable shortening of the red end of the spectrum will look at a red light (which is so dazzlingly bright to a normal-sighted person as to make his eyes ache after looking at it closely for a few seconds), at a distance of a few inches, and remark that there is nothing visible, and that the whole is absolutely black. It is obvious that the light must consist only of rays occupying the missing portion of the spectrum. The same remarks which I have made for a shortened spectrum apply to cases in which there is defect of light-perception through absorption or any other cause. The person having the defect is placed in a similar position to a normal-sighted person with those particular rays removed or reduced to the same intensity. Another effect of shortening of the spectrum when it is sufficient to interfere with the difference-perception which appears to be inherent in the central nervous system, is that the colours appear to be moved in the direction of the unshortened portion. For instance, we find the neutral point of the dichromic, with shortening of the red end of the spectrum, in most cases further towards the violet end of the spectrum in comparison with a case in which the spectrum is of normal length. In the same way a trichromic with a shortened red end of the spectrum has the junction of the red and green nearer the violet end than in a case where there is no shortening. The point that I specially wish to emphasise is that, though every case in which there is defective light-perception can be explained by a defective sensibility to light of certain wave-length, not a single case of the very large number of persons that I have examined can be explained on the older theories; that is, the defect of light-perception cannot be explained on the assumption that there is a defect in a light-perceiving substance which is sensitive to rays of light from a considerable range of the spectrum. A large number of cases in which there is shortening of the red end of the spectrum escape detection when only the green test is used, as is usual according to Holmgren’s instructions.