We next observe the organs of motion with which the eye is furnished, and which, with complicated strings and pulleys, can turn it every way at the will of the intelligent agent. The _pupil_ or _opening_ of the eye, also, is so constructed, with its minute and multiplied circular and crossing muscles, that it can contract or expand in size just in proportion as the light varies in intensity. The ball of the eye is filled with three substances of different degrees of density. One is a watery humor, near the front of the eye; back of this, and suspended by two muscles, is the solid lens of the eye, or the _crystalline humor_; and the remainder of the eye, in which this lens is imbedded, consists of the _vitreous humor_, which is of the consistence of jelly. These all have different degrees of transparency, and are so nicely adjusted that the rays of light, which start from every point in all bodies in _diverging_ lines, are by these humors made to _converge_ and meet in points on the _retina_, or the nerve of the eye, forming there a small picture, exactly of the same proportions, though not the same size, as the scene which is spread before the eye. When the outer covering of the back part of the eye is removed, the objects which are in front of the eye may be discerned, delicately portrayed in all their perfect colors and proportions, on the retina which lines the interior. It is this impression of light on the optic nerve which gives our ideas of light and colors. The eye is also formed in such a way that it can alter its shape and become somewhat oblong, while at the same time its lens is projected forward or drawn back. The object of this contrivance is to obtain an equally perfect picture of distant and of near objects. Our ideas of _shape_ and _size_ at first are not gained by the eye, but by the sense of touch. After considerable experience we learn to determine shape and size by the eye. Experiments made upon persons born blind and restored to sight furnish many curious facts to support this assertion. When the eye first admits the light, all objects appear to _touch_ the eye, and are all a confused mass of different colors. But by continual observation, and by the aid of the sense of touch, objects gradually are separated from each other, and are then regarded as separate and distinct existences.