2. _Defects of colour-perception._--The colour-blind have a diminished hue-perception and see a less number of colours than the normal. All the symptoms of colour-blindness are such as we should expect from want of development of the retino-cerebral apparatus for the perception for colour. This is evident even in the slighter cases which show a diminished colour-perception compared with the normal. We find that the colour-blind are much more dependent on the luminosity of the colour than the normal-sighted; they require a stronger stimulus; they fatigue more easily with colours than the normal-sighted; they have a more marked simultaneous contrast; the visual angle subtended by the coloured object requires to be larger, and they have a very bad memory for colours. The diminution of colour-perception with a diminished visual angle evidently depends upon several causes. It is very marked when there is diminished light-perception for those rays which are imperfectly seen. It is also dependent upon certain retinal conditions, as in scotoma and allied conditions. There are colour-blind persons, however, who are able to recognise colours under as small a visual angle as the normal-sighted, and I have examined one dichromic (said to be red-blind by a physicist) who recognised red easily through the thickest neutral glass of my lantern, and who had no difficulty with this colour at a distance. Apart from any other defect of light or colour-perception, every case with which I have met has fallen naturally into one of the classes I have given; that is to say, every person is either heptachromic, hexachromic, pentachromic, tetrachromic, trichromic, dichromic, or totally colour-blind. When I first gave this classification of colour-blindness, the facts that I discovered were so at variance with those generally stated that it was very difficult for those who were not well acquainted with the subject to compare the two sets. The general knowledge of the subject has, however, steadily increased, and the facts which I had so great a difficulty in getting recognised now form part of our common knowledge. It would be well, therefore, to describe the two main varieties of colour-blindness which are of chief practical importance, and to show the relation which they bear to the writings of other persons. These two main varieties have dichromic and trichromic vision.