AN ACCOUNT OF SOME OBSERVATION TESTS AND OF A NEW 



STEREOSCOPE. 



BY JOHN F. SHEPARD. 



(Abstract.) 



In the tests to be used for observers in the aviation service, the individual 

 was given a photograph of a large and varied landscape and allowed to study 

 it twenty minutes. Of several tests tried, two which seemed to show signifi- 

 cant individual differences were retained for use. In the first, the subject 

 was shown for two seconds a detached piece of the picture, either on the same 

 scale as the original, or enlarged, or reduced, and on which either one or two 

 crosses were marked. The subject's problem was to grasp the crosses in rela- 

 tion to familiar landmarks, and, after the exposure, to refer to the original 

 and record the location of the crosses accurately with reference to coordinates. 

 The results were graded by counting 10 as perfect and deducting one point for 

 each eighth of an inch of error. This was based upon a plot of distribution of 

 errors which showed localizations with errors greater than about one inch to 

 practically pure guess. Sixteen pieces were shown in the test. In the second 

 test, the subject was shown a picture of the same character and size as the 

 original, of the same landscape except that it might be modified at as many 

 as six places. The modification was done by retouching with ijencil or KCN 

 solution as needed. Any objects might be added or deleted and replaced by 

 other conditions. The subject's problem was to detect and grasp accurately 

 any changes from the original. The original was not in sight during this 

 exposure. After the exposure the subject referred again to the original and 

 recorded the location and concise description of all changes he was able to 

 detect. Sixteen modified forms were shown in the test. Both tests showed 

 marked individual differences, but depend upon partly different factors, and, 

 though there is a strong correlation, a man's rank in the one is not necessarily 

 the same as his rank in the other. 



The stereo.scope Is a modification of the Wheatstone instrument with the 

 addition of appropriate convex lenses to be worn by the operator and arranged 

 to mount .simultaneously in the field of fixation two pairs of lai'ge airplane stere- 

 oscopic photographs. This gives a larger view than the Brewster and makes 

 possible advantageous comparison of two views of the same region. The 

 instrument allows independent adjustment of the different pictures. The 

 lenses compensate for the lack of convexity of the lenses of the eyes with 

 accommodation for distant vi.sion. and thus give a clear view and excellent 

 perspective. It was in this especially that the Brewster excelled the older 

 Wheatstone. 



University of Michigan. 



