And here I may remark that it has been assumed that the two classes of colour blindness are due to different causes. A question to ask ourselves is whether all colour blindness may not have been caused originally by disease. In the congenital form, it is true, no disease of the retina is traceable in the eye, and it is usually hereditary, but it does not follow that the want of response of the perceiving apparatus to certain sensations may not have been due to what, for want of a better expression, I may call an hereditary partial paralysis of the perceiving apparatus. If this be so, we have a connecting link between the two classes, and then a perfect theory should explain both classes on the same grounds. The suspicion that the monochromatic vision of P. and Q. might possibly be due to disease before birth, owing to the behaviour of their eyes under certain conditions, would then be explicable. I have no desire to press this view, though it seems to me to be one which is not out of all reason, taking analogies from other defects which are hereditary. It has been usually accepted that the fields for blue and yellow in the eye are approximately the same, as are those of the green and red, and this has been taken as showing the interdependence between the two pairs according to the Hering theory. It has already been pointed out that the question of extent of fields requires still further investigation beyond that which it has received, and measures made by the method given on page 208 seem to cast a doubt as to whether this interdependence can be upheld. It will be noticed that the fields do not extend proportionately on the nasal and temporal sides (see also Fig. 3). It should also be remarked that the order of extent of field for the different colours does not follow the same order as their disappearance. A point that is sometimes raised in favour of Hering’s theory is the negative image formed after the eye is fatigued by looking at bright red or bright green. The negative images (see page 30) are said to be the complementary of these colours. The Young theory tells us that the red or the green sensation suffers fatigue by one or other colour, and that when the eye subsequently rests on a grey surface the other two sensations are chiefly stimulated and cause the complementary colour. It is said that it is easier to produce a _negative green_ image than a _negative red_ image, and the adherents of Hering tell us that this is due to the fact that destructive action is more readily carried out than constructive. In the Young theory, it is held that the green sensation is always mixed with white, whilst the red is fairly pure, and thus, for equal luminosities, the surplus green sensation is much less stimulated than the red, which offers a consistent explanation of this fact. There are several other minor difficulties in the way of accepting Hering’s theory as it stands from a physical point of view, but we need not discuss them now.