Whether R., G., and V. are all three present in each cone, or whether they are each developed in separate cones, we do not know for certain. Nor are we certain that there are separate nerve-fibres for the transmission of stimuli due to R., G., and V. When we look steadily at a red object we cause the disruption of R.; and since it takes some time for the reformation and reconstitution of this explosive substance, on turning the eye to a grey surface, G. and V. are alone, or in preponderating proportions, caused to undergo disruption. Hence the phenomena of complementary after-images. It is not merely a matter of the tiring of certain nerve-fibres, but a using-up of the explosive material in certain of the cones. What is called _colour-blindness_ is probably due to one of several abnormal conditions. It is _possible_ that in some cases R., G., or V. may be entirely absent. More frequently they are in abnormal proportions. They probably vary in their sensitiveness, and not improbably in the wave-period to which they show the maximum response. To test the variation, if any, in the limits of instability for R. and V., or in any case in the limits of colour-vision at the red end and at the violet end of the spectrum, in apparently normal individuals, my friend and colleague, Mr. A. P. Chattock, made, at my suggestion, a number of observations on some of the students of the University College, Bristol, to whom my best thanks are due for their kind willingness to be submitted to experiment. The instrument used[FF] was a single-prism spectro-goniometer. In the accompanying diagram (Fig. 33) the results of some of these observations are graphically shown. The middle part of the spectrum, between the wave-lengths 420 and 740 millionths of a millimetre, is omitted, only the red end and the violet end being shown. The observations on thirty-four individuals, seventeen men and seventeen women, all under thirty years of age, are given for both eyes. The left-hand vertical line of each pair stands for the right eye in each case. To the left of the table are placed the wave-lengths in millionths of a millimetre. [Illustration: Fig. 33.--Variation in the limits of colour-vision.] Take, for example, the first pair of vertical lines. The individual whose colour-range they represent could detect red light in the spectrum up to 800 millionths of a millimetre wave-length for the right eye, and up to 811 for the left; and could detect violet light down to 403 and 404. Beyond these limits all was dark. But the last individual in the series, while his range in the violet was about the same, could only detect red light up to 743 and 750 millionths of a millimetre. His spectrum was so much shorter.