470 THE POPULAR SCIENCE MONTHLY. 



a wide slit, it affords a view over a greater part of the eyeball, and we 

 think it bigger, simply because we see more of it. In the same man- 

 ner, our judgment is deluded by the different degrees of prominence 

 of the eye. A staring or protruding eye impresses us as being larger, 

 although it is only pressed forward ; while in advanced age, or in con- 

 suming sickness, the sunken eye is thought smaller. 



If the eye really is larger, then the distance between the cornea 

 and the lens will be greater; and if the effects of the refracted light re- 

 main the same in the latter, the image will no longer be projected on 

 the cornea, but in front of it. And this is what really takes place in that 

 wide-spread malady called short-sight dness. Here we have especially 

 to note that the mean axis of the eye is too long. There are others 

 called far-sighted eyes, whose visual axis is too short, the image for 

 such eyes falling behind the retina. In order to reestablish the condi- 

 tions of keen vision in both cases, the effects of refraction must be di- 

 minished for the short-sighted, by diverging or concave glasses ; for 

 the far-sighted, by collective or convex glasses. Those conditions have 

 nothing to do with the want of the power of adjustment of focus. 

 If you correct the defective construction of the short-sighted eye with 

 a concave glass, and that of a far-sighted with a convex one, the lens 

 its mobility being preserved can with their aid accommodate it- 

 self to near and distant objects, which neither an old man, deprived 

 altogether of the power of adjustment, nor an individual who has been 

 operated on for the cataract, is capable of doing. 



Let us now pursue the analysis of the model in the same order as 

 at the outset. First, then, fold back the cornea with the anterior sec- 

 tion of the sclerotica ; the margin of the eye to the front is now inter- 

 rupted by the pupil, and is for the rest formed by the tissue of the iris, 

 and the anterior section of the choroid. The space (at present want- 

 ing) in front of the iris-curtain was filled with aqueous humor, which 

 you are to suppose has escaped. Let us now remove the posterior 

 half of the tissue of the visual nerve ; the whole eye will then be closed 

 up by the passage of the choroid and iris, which now meets with no 

 break, except anteriorly from the pupil, and posteriorly from the en- 

 trance of the optic nerve. If, now, as with the sclerotica, we fold back 

 the anterior division of the choroid along with the iris, in doing which 

 we have an opportunity of convincing ourselves of the true nature of 

 the pupil, and that it is indeed an opening, we then come upon the 

 hard lens lying behind. Taking this away, as also the gelatinous vit- 

 reous humor, we at length remove the posterior section of the choroid, 

 and have nothing left but the optic nerve and the retina. Thus we 

 have again arrived at the starting-point of our reflections ; and it only 

 now remains for me to bring before you the retina. 



Before, however, entering on this division of my subject, let me 

 call your attention to a few fundamental processes in the act of seeing. 

 First, the picture or image on the retina is perfectly sharp only at one 



