For what I may call the straightforward part of colour vision, dealing with ordinarily bright colours, the Young theory is amply sufficient; but when we come to the feeble luminosities and the colour fields, it is again difficult to adapt to explain the phenomena observed. When we reduce the luminosity of a coloured ray sufficiently we feel the sensation of grey light: no colour is felt. Why is this? On the Hering theory it is capable of the explanation that we have the white sensation left unextinguished, but I fail to see any explanation on the Young theory. When we take colour fields with pure colours (see appendix, page 208), we are met with the unexplained difficulty that the colour from a bright spot of light vanishes almost suddenly towards the periphery of the retina, and is replaced by a bright _white_ light, and that the extent of the field depends on the brightness of the colour. This, perhaps, is the most telling observation which can be recorded against the Young theory as it stands at present. It has this support, however, in the _sequence_ of the phenomena observed, viz., when the boundary for the colour which we will suppose to be pure red is being taken (as described at page 11), that close to the point where it bursts into pure white, it assumes a pink colour (_i.e._, a mixture of red and white), whilst, if the red be scarlet, containing according to this theory a little green sensation, it becomes orange before white, showing that the red sensation is dimmed slightly before the green, and so with the other colours. What are called “after images” I have not touched upon so far, nor shall I here, for it is at this point that we step into very debateable ground. The colours perceived in them are, as yet, not capable of being put to the test of physical measurement, and I must leave the psychologist or the physiologist to account for them in their own way.