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PEACTICAL PHYSIOLOGY 



6. The Protrusion of the Lens during Accommodation for near 

 objects. The actual bulging forward of the lens during accommodation 

 for near objects may be seen in the following experiment. 



EXPERIMENT. Using only one eye, let the subject of the experiment 

 have arranged in the optic axis two objects, one about ten inches, the 

 other distant. The observer takes a position so as to view the subject's 

 eye in profile, but somewhat obliquely, so that he can just see the 

 further side of the iris. When the subject's eye is accommodated for 

 the near object, more of the pupil shows and the further side of the iris 

 becomes narrower. This is obviously the result of bulging forward 

 of the lens. This is independent of the variation of the diameter of 

 the pupil for the different objects. 



FIG. 95. Diagram of the course of the rays of light in the phakoscope. 



7. Schemer's Experiment. If the eye be accommodated for an 

 object at any particular distance, the effect of preventing the retina 

 receiving all the rays from the object (as by a screen with holes 

 pricked in it and held close to the cornea), is simply to diminish the 

 brightness of the image, on account of the lessening of the amount 

 of light entering the eye. Any object at a distance for which the eye 

 is not accommodated will form a blurred image on the retina, and if rays 

 from the object by this partial screening of the retina have several paths 

 by which to impinge on the retina, there will be formed upon the 

 retina as many blurred images as there are openings in the screen. 

 When, however, the eye is accommodated for this second object, these 

 blurred images fade into one clear image. 



EXPERIMENT I. To form a screen take a thin piece of cardboard and 

 prick two holes in it, separated by less than the diameter of the pupil. 

 About one-sixteenth of an inch will answer. Place in a strip of wood 

 about a yard long two vertical needles, distant eight and twenty-four 

 inches from the eye. Close one eye and with the other, holding tho 

 screen close to cornea, look at one of the needles. The other needle 



