SCHEINER S EXPERIMENT. 



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the screen. If the dioptric apparatus of the eye were rigid and 

 unalterable, since the relation of the retina to it is permanently 

 the same, we could only see those objects clearly which are at a 

 given distance from the eye. We know, however, that we get a 

 distinct image of distant as well as of near objects, and we can 

 look through the window at a distant tree, or we can adjust our 

 eyes so as to be able to see a fly walking on the window pane. 

 However, we cannot see both distinctly at the same moment. 

 This may be demonstrated by what is known as Schemer's ex- 

 periment, which is carried out in the following way : Two pin 

 holes are made in a card at a distance from each other not wider 

 than the diameter of the pupil. The card is then brought close 

 to the eye, so that a small object such as the head of a bright 



FIG. 224. 





To illustrate Scheiner's experiment; for explanation, see text 



pin can be seen through the holes. The dioptric media being 

 fixed, moving the object nearer to or further from the eye would 

 have the same effect as changing the relation of the retina to m n 

 or p q in Fig. 224, by means of which we may explain the follow- 

 ing observations : (1) The eye being fixed upon the object (of 

 which only one image is seen), move the pin rapidly away ; two 

 objects now appear, showing that the rays coming through the 

 holes have met before they reach the retina, as at p q. (2) Move 

 the pin near the eye ; again two very blurred objects are seen, for 

 the rays have not met when they strike the retina, as at m n. (3) 

 Keeping the object in the same position, alter the gaze, as if to 

 look first at distant and then at near objects ; in both extremes 

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