We have already discussed this: wherever the seat of the injury in the sensory routes and whatever its origin, one observes, for example, a decay of sensitivity to colour; at the beginning, all colours are affected, their basic shade remaining the same, but their saturation decreasing; then the spectrum is simplified and reduced to four colours: yellow, green, blue, crimson, and indeed all short wave colours tend towards a kind of blue, all long wave colours towards a kind of yellow, vision being liable, moreover, to vary from moment to moment, according to degree of fatigue.