We arrive at this by simple calculation. Supposing we have two luminosities, _one double the other_, it does not require much thought to find out that you have to reduce the greater luminosity twice as much as the other in order for it to be just extinguished. In other words, if we multiply the extinction by the luminosity, we get what we want. Now, in the curves before us, we have taken the luminosity of the yellow light near D as one amyl-acetate lamp, and that has a height in the curve showing the spectrum luminosity very closely approaching 100. We may, therefore, multiply the extinctions of a ray by the value of its ordinate in the luminosity curve and divide the result by 100, and this will give us the extinction of each colour, supposing it had the luminosity of an amyl-acetate lamp. A portion of the curve so calculated is shown in the same diagram (Fig. 28) as a dotted line. It appears at the violet end as an approximately horizontal line, and then starts rapidly upwards, and would, if carried on to the same scale, reach far out of the diagram; but at the extreme red it would be found to bend and again become horizontal. I would have you notice that the same is true not only for the extinction observed with the centre of the eye through the yellow spot, but also for the whole eye. Such straight, horizontal parts of the curve must mean something. [Illustration: FIG. 30.] In the diagram (Fig. 16) of colour sensations we see that in each of these two regions there is but one sensation excited, viz. the violet and the red. Now, if these sensation curves mean anything, the reduction necessary to produce the extinction of the same sensation when equally stimulated should prove to be the same, for there is no reason to the contrary, but exactly the reverse. _Primâ facie_, then, taking the Young theory as correct, we may suppose that these horizontal parts are due to the extinction of one sensation. Let us treat it as such, and go back to the original extinction curve shown in the continuous lines. The parts of the curve which lie over the fairly horizontal dotted line, at all events, should be the extinction curve of the same sensation, but more or less stimulated or excited. As before explained, if we have double the stimulation at one part of the spectrum to that we have at another, the reduction of the greater luminosity to give extinction will be double that of the lesser. If, then, we take the _reciprocals of the extinction_, it ought to give us a curve which is of the form of some colour sensation; and when we arrive at the maximum, we may for convenience make that ordinate 100, and reduce the other ordinates proportionally. This has been done in Fig. 30 in the curves C and D. For the sake of a name my colleague and myself have named such curves “persistency curves.” Perhaps some other name might be more fitting; but still a poor name is better than none at all.