If a similar examination be made of the green-blind, the red end of the spectrum will be called red or yellow, but the spectrum itself will be visible between the same limits as it is to the person who has the normal sense of vision. A grey stripe will be seen in the spectrum, but in this case it will be a trifle nearer the red end of the spectrum than the point which the red-blind calls grey; from this point to the extreme violet, the green-blind will name the spectrum colours similarly to the red-blind. The part of the spectrum where grey exists to the green-blind is even more important than that part at which it exists to red-blind, for it marks the place where the red and violet curves cut each other in Fig. 16, and is in the majority of cases the place in the spectrum where to the normal eye the green sensation is unmixed with any sensation except that of white, as quite recently explained. This green evidently is the colour which is most usefully employed in making colour mixtures in order to obtain the three sensation curves of the Young theory, since white can be added to the colour matched. To avoid verbiage, we shall call the points where the red- or green-blind see a grey in the spectrum their neutral points, and the grey they see at those points their neutral colours. The three curves we shall call the red, green, or violet curves, and the slits, when placed in the red, green, or violet of the spectrum, as the red, green, and violet slits. We have already mentioned the case of those who possess monochromatic vision, and shown in what respect they will differ in their description of the spectrum from those more common cases of defective vision. If the visual sensation they possess be the violet, they will see no light at the extreme red of the spectrum, and very little in the orange. They must match every colour with some shade of grey, for they will only perceive that sensation, in what to ordinary normal eyes is white. We need not detail how those who possess monochromatic vision due to some other sensation would describe the different colours. The diagram will tell us. Suffice it to say, that one colour will only differ from another and from white in brightness.