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PRACTICAL PHYSIOLOGY 



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

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



Fio. 214. Diagram of the course of the rays of light in the phakoscope. 



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 the 

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

 will be also seen, but represented by a double blurred image. If the 

 more distant needle be accommodated for, a double blurred image of 

 the nearer will be obtained. Cover one of the holes in the screen 

 with another card. If the right hole be covered the left blurred 

 image will disappear, and conversely. Let the eye be now accommo- 

 dated for the nearer image. A double blurred image of the more 

 distant needle will be seen. If the right hole of the screen be now 

 covered the right blurred image will disappear, and conversely. 



EXPERIMENT II. A slight modification of this experiment and the 

 material requisite is provided in the Milton Bradley Pseudoptics, 

 Section I., exp. 4. 



