As the hole H is supposed to be so small as to receive only one ray from every point of the object, the images of the object, viz., _br_, _b′r′_, _b″r″_, will be very faint. By widening the hole H, so as to admit more rays from each luminous point of RB, the images would become brighter, but they would become at the same time indistinct, as the rays from one point of the object would mix with those from adjacent points, and at the boundaries of the colours R, Y, and B, the one colour would obliterate the other. In order, therefore, to obtain sufficiently bright images of visible objects we must use _lenses_, which have the property of forming distinct images behind them, at a point called their focus. If we widen the hole H, and place in it a lens whose focus is at _y_, for an object at the same distance, HY, it will form a bright and distinct image, _br_, of the same size as the object RB. If we remove the lens, and place another in H, whose focus is at _y′_, for a distance HY, an image, _b′r′_, half of the size of RB, will be formed at that point; and if we substitute for this lens another, whose focus is at _y″_, a distinct image, _b″r″_, _twice_ the size of the object, will be formed, the size of the image being always to that of the object as their respective distances from the hole or lens at H. With the aid of these results, which any person may confirm by making the experiments, we shall easily understand how we see external objects by means of the images formed in the eye. The human eye, a section and a front view of which is shewn in Fig. 5, A, is almost a sphere. Its outer membrane, ABCDE, or MNO, Fig. 5, B, consists of a tough substance, and is called the _sclerotic_ coat, which forms the _white of the eye_, A, seen in the front view. The front part of the eyeball, C_x_D, which resembles a small watch-glass, is perfectly transparent, and is called the _cornea_. Behind it is the _iris_, _cabe_, or C in the front view, which is a circular disc, with a hole, _ab_, in its centre, called the _pupil_, or _black of the eye_. It is, as it were, the _window_ of the eye, through which all the light from visible objects must pass. The _iris_ has different colours in different persons, _black_, _blue_, or _grey_; and the pupil, _ab_, or H, has the power of contracting or enlarging its size according as the light which enters it is more or less bright. In sunlight it is very small, and in twilight its size is considerable. Behind the iris, and close to it, is a doubly convex lens, _df_, or LL in Fig. 5, B, called the _crystalline lens_. It is more convex or round on the inner side, and it is suspended by the _ciliary_ processes at LC, LC′, by which it is supposed to be moved towards and from H, in order to accommodate the eye to different distances, or obtain distinct vision at these distances. At the back of the eye is a thin pulpy transparent membrane, _rr_ O _rr_, or _vvv_, called the _retina_, which, like the ground