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LIGHT SCIENCE FOR LEISURE HOURS 



apparent brightness of the part of the water just 

 below the moon, where lies seemingly l a broad track of 

 silver light. If this track of light is concealed in any 

 way, as by holding up a sheet of card or paper, the 

 portion of the sky immediately above is at once seen 

 to be at least as bright as the parts of the sky on either 

 side of it. So in multitudes of other cases, some 

 familiar, some otherwise the eye is deluded by effects 

 of contrast. 



Photography, or what may be called photographic 

 vision, is not, it is true, altogether free from defects 

 corresponding to such defects of vision (resulting in 

 illusion) as we have just considered. As there are 

 physiological illusions in ordinary vision in such cases, 

 so are there in certain applications of photography, 

 physical effects which may prove similarly illusive. 

 For instance, there is what is sometimes called photo- 

 graphic irradiation, when around a dark object in a 

 photograph a ring of light is seen, or around a bright 

 object a ring of darkness, this ring not corresponding 

 to any really existent object, but resulting from some 

 change in the photographic film along the border-line 

 around a region acted on very strongly by light. 



Again, the photographic eye has long been justly 

 valued for its artistic power, in being able to record, 

 without defect or exaggeration, what it sees. If we 



1 I say ' apparent ' and ' seemingly ' because the moon's rays really 

 illuminate the region which appears dark, as brightly as the rest. It 

 is only because of the position of the observer's eye that one region 

 appears brighter than the rest. 



