VISUAL JUDGMENTS 227 



receives a slightly different picture, and we are furnished with a 

 most important means of judging solidity and distance. Objects 

 seen with one eye only have a very flat appearance. Shadows 

 help materially in conveying impressions of solidity, and their 

 significance may be illustrated in the interpretation of aeroplane 

 photographs. In such photographs taken during the War there 

 were numerous marks which might have either a positive or a 

 negative solidity, i.e. they might have been either projections 

 above the ground, such as machine-gun emplacements, or depres- 

 sions, such as shell holes. This could only be determined by the 

 shadows cast, and the direction of the sun's rays was always marked 

 on such photographs. Thus A is a projection and B a depression. 



In judging very short distances, two ^ 



eyes are an enormous advantage. Try 

 to thread a needle with one eye closed. ^ ^ 

 At rather longer distances this may 0. ] 

 be demonstrated by looking at a wall ^%^ 

 between which and the observer an A B 



object, such as the wire of an electric Fl - 54 ~ Shadows - 



lamp, is suspended. Using one eye only and avoiding looking 

 at the point of attachment to the ceiling we will judge its distance 

 from the wall very imperfectly, but with both eyes we can make 

 an accurate estimation. 



At long distances numerous external factors come into play. 

 Perspective, light and shade and atmospheric conditions are of 

 importance. Thus " visibility " may be good or bad, and will 

 influence our judgments. At sea, where the surface is perfectly 

 flat and the gradations of illumination change uniformly with 

 distance, the untrained eye commits the grossest errors. The 

 proximity of an object of known size frequently supplies a scale 

 against which to measure the size and consequently the distance 

 of unknown distant objects. The faculty of judging distances is 

 poorly developed in the average man. A trained soldier or a 

 big game shot can make incomparably more accurate estimations 

 of distances for rifle fire than the novice. The same thing occurs 

 with the expert golfer for certain distances. 



5. The Stereoscope. The combination of two slightly dissimilar 

 pictures to form an apparently solid object is illustrated by the 

 stereoscope. This instrument consists of two prisms or half 

 lenses placed, with the thin edge inwards, about the same dis- 

 tance apart as the eyes. The dissimilar pictures P 1 and P z are 

 fixed one in front of each lens, and a median screen cuts off each 



