ELEMENTARY EXPERIMENTAL PHYSIOLOGY 105 



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. 



EXPERIMENT III. Experiment I. can be most instructively performed 

 with Kiihne's Artificial Eye. A special screen for the experiment 

 is provided in which one hole is covered with red mica. Accommo- 

 dation for the different distances is provided by shifting the retinal 

 screen backwards or forwards, and the illuminated arrow can be used 

 as an external object. It is found that if the screen be shifted forward 

 so as to accommodate for objects beyond the arrow, that two blurred 

 images of the arrow obtain. Covering either hole will block either 

 image. But when the eye is accommodated for a more distant object it 

 will be observed that covering the left hole removes the left retinal 

 image. If the images be projected, as before, on the lantern screen, 

 the opposite image will of course be removed. The apparent contradic- 

 tion between Experiments I. and III. is obviously due to the fact that 

 in I. the images are referred to the field of vision, in III. (without the 

 use of further projection on the lantern screen) they are actually viewed 

 as formed on the retina. 



EXPERIMENT IV. The near point of accommodation can be 

 conveniently ascertained by noting the least distance at which a single 

 image of a needle can be seen, when using the perforated screen of 

 Scheiner's experiment. 



EXPERIMENT V. In Experiment II. note that the thread on which the 

 needle hangs remains clear as a single thread for a certain distance on 

 either side of the needle, but that beyond this distance it gradually bifur- 

 cates into a double thread. This singleness of the thread corresponds 

 to the length of the line of accommodation. (See Experiment II, 

 page 101.) 





