206 ANATOMY AND PHYSIOLOGY. 



-egg, and is enclosed in a very fine transparent membrane, ramifying 

 also into its interior, as in the foregoing figure. 



By the united action of all these parts, vision is produced. The 

 cornea serves the purpose of a convex or magnify ing -glass, to col- 

 lect into foci or points the rays of light that pass from an object to 

 the eye, and this effect is still further assisted by the lens placed 

 behind it. The point where these foci are thus formed, is the 

 retina ; and the eye may be compared to the optical instrument 

 called the camera obscura, which is, indeed, but an imitation of th 

 eye itself. Those who have seen this instrument will know, that 

 when the part corresponding to the cornea is presented to a land- 

 scape, there is an exact picture of it formed on the back part of the 

 box. Kepler, the great astronomer, made the interesting discovery 

 that the same thing may be seen in the eye. If the eye of a 

 recently killed bullock be carefully stripped of its sclerotic and 

 choroid coats posteriorly, and the retina be supported by a piece of 

 transparent silk, it may be placed in the hole of a window shutter 

 looking out upon a landscape, and a diminutive but distinct picture 

 of the whole may be seen depicted on the retina. From the thin- 

 ness of the coverings of the eye in albino animals (such as the white 

 rabbit), this exquisitely beautiful experiment may be performed even 

 without removing any of the coats. 



It is truly wonderful to think that all the accurate perceptions of 

 this sense are derived from the images of a crowded picture formed 

 at the bottom of the eye, on a space so small that it may be 

 covered with the point of the finger. What can be more astonish- 

 ing than the fact, that the image of the sail of a windmill, six feet 

 in length, seen at the distance of twelve paces, occupies only the 

 twentieth part of an inch on the retina, and that the image of the 

 same sail, when removed to the limits of distinct vision, occupies, 

 according to the calculations of M. de la Hire, only the eight 

 thousandth part of an inch, or less than the sixtieth part of the 

 breadth of a common hair! " We can never," to quote again Dr. 

 Paley's words, " reflect without wonder upon the smallness yet 

 correctness of the picture formed at the bottom of .the eye. A 

 landscape, of five or six square leagues, is brought into a space of 

 half an inch diameter, yet the multitude of objects which it contains 

 are all preserved are all discriminated in their magnitudes, posi- 

 tions, figures, colors. A prospect is compressed into a compass of a 

 sixpence, yet circumstantially represented. A stage-coach, travel- 

 ling at its ordinary speed for half an hour, passes in the eye over only 



