The retina consists essentially of an inner and outer wall, enclosing matter which is similar to the grey matter of the brain. On the inner wall are the vessels which are connected with the optic nerve. The outer wall is epithelium coloured with a pigment, and it is here that the visual impulses begin, although the rays of light giving rise to them have to pass through the thickness of the retina before so doing. It has already been stated that the light has to pass through the thickness of the yellow spot before a visual sensation is felt in the centre of the field, and the experiments just given offer a fair proof of the truth of the assertion, but there is still another which is perhaps more conclusive. Suppose we have a hollow reflecting ball, as shown in Fig. 2, and through an orifice A we project a beam of light to B, which meets an obstruction, S, in its path, then A B would be reflected from B to C on a screen C F, and the obstruction S would be marked at C. If another beam from D was directed so as to meet the same obstruction, its presence would be marked at F. Knowing the distance of the centre O of the hollow sphere from F C and its diameter, and measuring the distance between F and C and their respective distances from the axis of the sphere, the distances S B and S E can be calculated. This method is applied in the formation of what are known as Purkinje’s figures. The simplest case is where a beam of light is directed through the sclerotic and transmitted through the lens. Images of the retinal vessels are distinguished as at S, and it is found that they cast shadows, which are seen as dark lines in the glare of the field of vision. The sensation of light must therefore come from behind these vessels, and calculation shows that the seat of the sensation is close to the pigmented inner wall of the retina.