. Human physiology : designed for colleges and the higher classes in schools, and for general reading. nerve of sight on the surface of the retinais arranged in papilla;, just as the terminations of the nervesof touch are. In Fig. 167 is represented a portion of the retina of a frog magni- fig. 167. fied three hundred times. The upperrows of papillae, which are without dots,are seen sideways. 445. The superiority of the eye, as anoptical instrument, is seen in a strikingmanner in several particulars, in whichdifficulties and defects to which all opti-cal instruments are liable are removed. The
. Human physiology : designed for colleges and the higher classes in schools, and for general reading. nerve of sight on the surface of the retinais arranged in papilla;, just as the terminations of the nervesof touch are. In Fig. 167 is represented a portion of the retina of a frog magni- fig. 167. fied three hundred times. The upperrows of papillae, which are without dots,are seen sideways. 445. The superiority of the eye, as anoptical instrument, is seen in a strikingmanner in several particulars, in whichdifficulties and defects to which all opti-cal instruments are liable are removed. There is, for example,a defect in the operation of lenses in optical instruments, whichis termed spherical aberration. This can be explained on , which represents a lens, L, I/, with some of the rays asthey pass through it. Now the rays R, R, R, are broughtto a focus at F; while the rays R, L and It, 1/ come to afocus much nearer, at I. It was found by experiment, that if thecentral portion of the lens be covered, so that the rays R, RR, cannot pass, a distinct image will be formed on a screen. THE EYE. 297 Pphericnl and chromatic aberration. How remedied in the eye. fmt at I. And, on the other hand, if the outer portion of theens be covered, so that the outer rays are intercepted, then themiddle rays, B R R will form an image on a screen at if the whole lens be used, no distinct image is formed, wher-ever you may place the screen. If you place it at /, it willreceive with the rays that come to a focus there, rays that havetheir focus at F. And so of other points. 446. It is in view of such experiments, that a contrivancehas been adopted in the construction of telescopes and micro-scopes, for the purpose of remedying the difficulty above de-scribed. What is called a diaphragm, or stop, is put in againstevery lens. It is a perforated partition which permits the lightto pass only through the central portion of the lens. The linesD, D, in Fig. 108, cutting off all rays i
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