This extinction of colour is one which often occurs, but is seldom noticed. The figure tells us that the orange is about the last colour of the spectrum left, some of the others still appearing as greys. The next to retain its colour is the green, and the most rapid to lose them are the red and violet. It must not be supposed that the colours remain of the same hue up to the time that they vanish. Pure spectrum red (red sensation) remains the same up to the last, but the scarlet becomes orange, and the orange yellower, and the green bluer. This is what would be predicted from the Young theory if the order of extinction of sensation be red, green, violet. This we shall see is the case. At nightfall in the summer the order of disappearance of colour may often be seen; orange flowers may be plainly visible, yet a red geranium may appear black as night; the green grass will be grey when the colour of the yellow flowers may yet be just visible. An early morning start in the autumn before daybreak will give an ample opportunity of satisfying oneself as to the order in which colours gradually re-appear as daybreak approaches. Red flowers will be at the outset black, whilst other colours will be visible as grey. As more light comes from the sky the pale yellow and blue flowers will next be distinguished, though the grass may still be a nondescript grey. Then, as the light still increases, every colour will burst out, if not in their full brilliance, yet into their own undoubted hue. CHAPTER IX. Not only, however, may we lose a sense of colour, but we may also lose all sense of light by reducing the energy of the different rays. We have seen that colour goes unequally from the different parts of the spectrum. We may therefore prognosticate that the light itself may disappear more rapidly from some parts than from others. You will scarcely, however, I think, be prepared for the enormous difference which exists in the stages of disappearance of the grey of the reduced red and of that of the reduced green. [Illustration: FIG. 27.]