ELEMENTARY EXPERIMENTAL PHYSIOLOGY 257 



corresponding points of the two retinae, the sensations corresponding to 

 a single image result. 



EXPERIMENT I. Place on a stereoscopic slide, or on a sheet of 

 cardboard, red and green postage stamps at a distance from each other 

 equal to the interocular distance, and similarly arranged. Observe 

 these in the stereoscope, and the sensation of a single image of a black 

 or brown postage stamp will result. 



EXPERIMENT II. Perform the experiment in the Milton Bradley 

 Pseudoptics, Section K, Experiment III. The fusion of the two retinal 

 images gives the impression that one is looking through a round hole 

 in the hand. 



4. Binocular Perception of Relief. The perception of relief which 

 enables a judgment as to solidity to be formed depends upon the fact 

 that the two pictures presented to the retinae are not identical. The 

 amount of variation in the pictures will depend upon the interocular 

 distance and the propinquity of the objects. The first being con- 

 stant, it follows that a judgment as to solidity is more easily formed 

 in the case of near objects than distant objects. Similarly, a 

 judgment as to the relative distances of an object from the observer 

 depends upon the difference in position of an object with respect to 

 surrounding objects which exists in the two views presented to the 

 two eyes. 



The difficulty in forming a judgment as to the precise position in 

 space of an object when viewed with only one eye may be shown in the 

 following experiment. 



EXPERIMENT I. Stick a knife into the wall, and balance on the 

 handle a cork. The height from the ground should be about five or 

 six feet. Close the left eye, and, starting at about ten feet from the 

 wall with the right hand extended forward, walk rapidly to the cork, 

 and by a sweep of the hand attempt to remove the cork. A lack of 

 success will frequently attend the effort. 



It is seen from this experiment that it is difficult to locate any object 

 precisely in space when a single ocular view is alone obtained. 



On the other hand, if perfectly flat pictures be taken differing from 

 each other to the same degree as actual pictures presented to the two 

 eyes would differ, and if such flat pictures be combined by some form 

 of stereoscope, or by crossing the eyes, the resulting sensations will 

 correspond to a single picture on which the different objects are 

 differently projected into the space embraced by the picture, in which 

 the quality of depth is added to the flatness shown by each picture 

 separately. 



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