Colour vision is a subject which enters into the domains both of physics and physiology, and it is thus difficult for any one individual to treat of it exhaustively unless he be a Helmholtz, who was as distinguished in the one branch of science as he was in the other. I am not a physiologist, and at the most, can only pretend to an elementary knowledge of the physiology of the eye, but I trust it is sufficient to prevent myself from falling into any grievous error. I shall try and show you, however, that the subject is capable of being made subordinate to physical methods of examination. I must necessarily commence by a very brief description of those parts of the eye in which it is supposed the seat of vision lies, but in terms which are not too technical. As to the mere optical properties of the eye I shall say but little, for they are not necessary for my purpose, although more particularly adapted to mathematical treatment than the other properties I have to describe. The eye may be diagrammatically represented as in the figure which is supposed to be a horizontal section of it, the figure being reproduced from Professor Michael Foster’s Physiology. [Illustration: FIG. 1. _Scl_ is the sclerotic coat. _Ch_ the choroid coat, with _CP_ the ciliary process. _I_ is the body of the Iris. _R_ is the retina or inner wall. _PE_ the pigment epithelium or outer wall. _L_ the lens held by the suspensory ligament _sp.l._ _VH_ is the vitreous humour. _ON_ the optic nerve, _ox_ is the optic axis, in this case made to pass through the fovea centralis, _f.c._ ]