SCHEINER'S EXPERIMENT. 567 



It may be seen from the foregoing figure that if the retina, 

 which normally would lie at 2, were placed nearer the dioptric 

 apparatus, say at 1, or further from it, at 3, it would not meet the 

 exact point of the luminous cone, but would receive the rays either 

 before they came to a point or after they had diverged from it. 

 Thus indistinct rings of light would be seen instead of one lumi- 

 nous point, and an image would be blurred and indefinite. 



From this it follows that the eye, when quite passive, can only 

 get an exact image of bodies which are placed at a certain dis- 

 tance from it, just as, for any given state of a camera, only those 

 bodies in one plane come into focus and give a clear picture on 



FIG. 224. 



To illustrate Schemer's experiment ; for explanation, see text. 



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 

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



