The organ of vision, then, in us consists of an essential sensory membrane, the retina, with its delicate rods and cones; and an accessory apparatus for focussing an inverted image on to the sensitive surface of the retina. The surface is not, however, equally sensitive, or, in any case, does not give an equal power of discrimination, throughout its whole extent. This is seen in the experiment above described. When we look at the dot we see the coin, but not distinctly. The area of clear and distinct vision is, in fact, very small, constituting the yellow spot about 1/12 of an inch (2 millimetres) long, and 1/30 of an inch (.8 millimetre) broad. And even within this small area there is a still more restricted area of most acute sensibility only 1/120 of an inch (.2 millimetre) in diameter. Nevertheless, within this minute area there are some two thousand cones, the rods being here absent. In carefully examining an object we allow this area of acute vision to range over it. Hence the extreme value of that delicate mobility which the eye possesses--a mobility that is accompanied by muscular sensations of great nicety. We saw that the sense of touch in the tongue is sufficiently delicate to enable us to recognize, as two, points of contact separated by 1/25 of an inch (1.1 millimetre). What, in similar terms, is the delicacy of sight? At what distance apart, on the most delicate part of the retina, can two points of stimulation be recognized as distinct from each other? If the points of stimulation be not less than 1/6000 of an inch (.004 millimetre) apart, they can be distinguished as two. Below this they fuse into one. The diameter of the end of a single cone in the yellow spot is also about 1/6000 of an inch (.0045 millimetre). With regard to the mode in which the stimulation of the retinal elements is effected, we have no complete knowledge. Certain observations of Boll and Kühne, however, show that when an animal is killed in the dark the retina has a peculiar purple colour which is at once destroyed if the retina be exposed to light. If a rabbit be killed at the moment when the image, say, of a window, is formed on the retina, and the membrane at once plunged in a solution of alum, the image may be fixed, and an "optogram" of the window may be seen on the retina. The discharge of the colour of the retinal purple may be regarded as the sign of a chemical change effected by the impact of the light-vibrations. But in the yellow spot there seems to be no visual purple. It is, indeed, developed only in the rods, not in the cones. Here, probably, chemical or metabolic changes occur without the obvious sign of the bleaching of retinal purple. In the dusk-loving owl the retinal purple is well developed, but in the bat it is said to be absent.