curve of luminosity of the monochromatic vision, and such a resemblance can scarcely be fortuitous. As a matter of fact, in this we seem to have come to the final curve for low luminosities, and is almost the same as that observed when the spectrum is reduced to such an extent that it is colourless throughout, a condition that it can assume, as we shall see very shortly. When the spectrum is rather more luminous, it gives a curve of luminosity which is similar to that of the ordinary spectrum when measured by a red-blind person. Here, then, we have an indication that a person with normal vision passes through a stage of red-blindness, as the intensity is diminished before he arrives at absolutely monochromatic vision. [Illustration: FIG. 25.] This investigation is of practical as well as theoretical interest, as General Festing and myself quickly discovered when we first made it. The curious colour of a moonlight landscape is entirely accounted for by it. White light becomes greenish-blue as it diminishes in intensity, and the reds and yellows, being reduced or absent, are not reflected by surrounding objects. Hence, moonlight is cold, whilst the sunlight is warm owing to their presence. When measuring these low luminosities, the various colours will in a great measure disappear. Part of the spectrum will be of that peculiar grey which was shown you in the experiment with the incandescent light (p. 34). By further experiment it is possible to arrive at an approximate determination of the point where all colour vanishes from the different parts of the spectrum. We use the same apparatus (Fig. 24) as before, the only difference being that each of the sectors is movable during rotation. The apertures of those through which the colour passes are reduced till all colour on the screen just disappears, the point being arrived at by a comparison with the white, which is itself also reduced. The apertures of the first sector alone need be noted, and from these readings the diagram (Fig. 26) is made (for measures, see page 216). [Illustration: FIG. 26.]