50 ON SOME ASPECTS OF THE 



physiological response of the nervous system and the 

 psychological setting of consciousness thus tend to 

 deceive us. It is the remarkable characteristic of such 

 sensory deceptions that they still persist even when we 

 realize from other evidence that they cannot be in accord 

 with the fact. This is particularly the case in regard 

 to errors of visual perception. The sun or moon appears 

 to be very much larger when near the horizon than when 

 floating in the zenith, yet when the angular distance, 

 subtended by its diameter, is measured under the two 

 conditions there is no difference. The enlargement is 

 not a real one, nor even due to such optical effects of 

 the atmosphere as produce the familiar colour changes ; 

 it is an error of judgement arising from the fact that 

 when near the horizon earthly objects of known size and 

 at known distances are viewed at the same time, and 

 a comparison is permitted which the zenith position does 

 not allow. It is easy to prove this, for if the rising 

 moon be viewed through a small hole in a card, the 

 aperture being so fine that all earthly objects are 

 excluded, then it appears to have its zenith size. There 

 are, many examples of visual deceptions in which the 

 primary error is in the sense organ. Thus two lines 

 of equal length can be made to appear unequal by 

 putting arrow-heads on each end of one and arrow-tails 

 at each end of the other ; an arch can be distorted by 

 placing over one of its segments an upright bar; two 

 parallel lines can be bent in or out by placing an oblique 

 series of short parallel stripes across each line, the series 

 inclining in the opposite sense athwart each line. 



Nor are such deceptions limited to visual impressions ; 

 the feeling of contact due to pressure is increased if the 

 pressing object is cooled, decreased if it is warmed, so 

 that if two pennies, one hot, the other cold, are placed 



