As far as the perception of colour is concerned, the principal part of the eye which is not distinctly optical--_i.e._ for the production of images--is the retina, and this it will be seen is in reality an outcrop of the brain, the connection between the two being the optic nerve. Owing to this connection, it is not easy to determine where the seat of colour perception is located; but for the purpose of physical investigation this is not of first-rate importance, nor does it affect the discussion of rival theories except in a minor degree. There are other subsidiary adjuncts in the eye to which, however, I must call attention, as they have a distinct bearing on the apparent intensity of some colours and of the hue that mixtures of others are perceived. The first is what is called the “macula lutea,” or yellow spot, a spot which it may be assumed exists in every eye. It is horizontally oval in form, and is situated in the very centre of the retina, embracing some 6° to 8° in angular measure. It has a brownish or yellowish tint, and the retina at this part is slightly depressed, being bounded by a slightly raised rim. In the centre of this area the retina becomes very thin, having a depression about 1/100 of an inch or ·3 millimetres in diameter, which is named the “fovea centralis,” where it is said that vision is the most acute. This statement can be well credited when we come to consider where the seat of the stimulation of sensation lies. The colour which tints the yellow spot is strongest at the crater-like rim, and fades away centrally and peripherally, and is said to be wholly absent in the fovea centralis.