I have so far spoken only of normal, or physiological, colour blindness; a peculiarity, or defect, present at birth, and, as far as is at present known, irremediable, but not associated with any defect of the visual functions, or with any disease or any optical peculiarities. What the nature and seat of this defect may be--whether in the eye or in the sensorium--is at present unknown, although some of the characteristics of the deficiency in colour sensation, I believe, seem to indicate the existence of a special part of the brain endowed with the functions for perceiving colour. But cases are well known to medical men in which colour vision, normal to start with, fails in greater or less degree in connection with disease. This part of the subject is large and very complex, and requires for its full elucidation an acquaintance with the diseases and disorders of the eye. Many of the phenomena accompanying acquired colour blindness, however, are of great interest to the physicist in his study of colour vision, more particularly in regard to the test of the truth of any particular theory. Through the kindness of several medical men, and Mr. Nettleship in particular, I have had the opportunity of examining by the colour apparatus several types of colour blindness due to disease. One feature, common, I understand, to all, or nearly all, cases, is the presence of some disease of the optic nerve. Defective sight--from loss of transparency of the cornea, the crystalline lens, or other transparent parts of the eye--does not interfere with the perception of colour; nor is true colour blindness, as I am informed, well marked, if present at all, in disease limited to the choroid and retina (see Fig. 1). Even in cases of the disease of the optic nerve, medical authorities tell us that great differences exist in the amount of colour defect, and that _although the colour defect always goes along with some other serious visual loss, either of form, light, or field_, the relation between these several factors of the visual defect is by no means always the same, so far as can be judged by the tests commonly used by ophthalmic surgeons. They tell us that in some cases of disease of the optic nerve, colour vision when tested by the wool test, which will be described shortly, may be almost perfect, whilst the capacity for reading test letters of the alphabet may be extremely bad, and _vice versâ_. It seems that in some cases these discrepancies cannot be accounted for; but in others the facts can be explained by the limitation of the disease to certain fibres of the optic nerve. Thus, if those fibres which supply the yellow spot region of the retina are alone involved, direct, or central, vision will be much damaged both for form and colour, whilst a little further from the centre of the field, the visual functions in such a case are often quite normal. From what has been said in the opening chapters,