THE SENSE OF SIGHT. 



175 



sents : first, it is considerably smaller than the object; and secondly, it is in- 

 verted ; the houses in the landscape stand on their i-oofs, the persons on their 

 heads ; whatever in the object is to the right, in the image is to the left, and 

 vice versa.^ For the production of this small inverted image, the glass lens 

 placed in the anterior opening is employed, serving for the transmission of the 

 rays proceeding from the object, but at the same time diverting them from their 

 course in such manner that the rays proceeding from each single point of the 

 object, which in front of the lens diverged from one another, converge behind 

 it, so that at a certain point on the plane of the smooth tablet they again unite. 

 This refraction of the rays of light by the glass lens will be rendered more 

 clearly intelligible through an explanation of the formation of the image in the 

 eye. This organ fully corresponds to such a camera obscura ; imder this aspect, 

 then, let us consider its arrangement and the signification of its separate parts 

 with the aid of the annexed figure, which represents a horizontal section of the 



Fiff. 1. 



eye made directly through its centre. The eyeball is, in general, of globular 

 shape, the optic nerve being inserted, like a handle, somewhat towards the 

 side of the nose. The outer wall of the globe is, with a partial exception, 

 formed of a hard opaque white tunic S, called the sclerotic, (the white of the 

 eye,) which answers to the wooden wall of the camera obscura, and serves as a 

 solid protective covering. On its inner side lies a second tunic or coat, the 

 choroid, which is extremely rich in blood-vessels, and is of a dark-brown or 

 chocolate color ; it is composed of three layers, the inmost of which consists of 

 small cells, loaded with minute pigmentary granules, and so arranged with re- 

 gard to one another as to resemble a tesselated pavement. The design of this 

 dark coloring is the same with that of the black coat with which the inner wall 

 of the camera obscura, as well as of the telescope or microscope, is furnished, 

 namely: to prevent the dispersion of the rays of light and consequent disturb- 

 ance of the perceptions of sight, as will be hereafter explained. 



On the front surface of the eyeball we see, as it were inserted into the 

 sclerotic, the transparent cornea H, arched in the manner of a watch-crystal ; 

 this forms the foremost part of the wall of the eye, and is distinguished from 

 the rest of the integument by its transparency and greater curvature. Behind 

 the cornea, like the dial-plate behind the watch-crystal, lies a brown or blue or 

 gray-tinted membrane, the iris R, with a circular opening in its middle, the 

 pupil P. The iris is, in effect, only the forward part of the choroid. The in- 

 terval between the hinder surface of the cornea and the front surface of the iris 

 is the "anterior chamber" of the eye, filled "with a fluid called the aqueous 

 humor. Behind the iris, and contiguous to its hinder surface, is the crystal- 

 line lens K, a lenticular highly transparent body, which turns its front and 

 less curved surface to the cornea, its hinder and more convex surface to 

 the background of the eye. The interval between this lens and the rear of the 

 eye is filled with a perfectly transparent, albuminous fluid, of the consistence 

 of thin jelly, called the vitreous body. These cursorily described parts — the 

 coi-nea, the aqueous humor, the pupil, the crystalline lens, and the vitreous body 



