ON THE EELATION OF OPTICS TO PAINTING. 99 



equal to that % of a candle burning at a distance of 12 

 feet. You know that we cannot read by the light of the 

 full moon, though we can read at a distance of three or 

 four feet from a candle. Now assume that you suddenly 

 passed from a room in daylight to a vault perfectly 

 dark, with the exception of the light of a single candle. 

 You would at first think you were in absolute darkness, 

 and at most you would only recognise the candle itself. 

 In any case, you would not recognise the slightest trace 

 of any objects at a distance of 12 feet from the candle. 

 These however are the objects whose illumination is 

 the same as that which the moonlight gives. You 

 would only become accustomed to the darkness after 

 some time, and you would then find your way about 

 without difficulty. 



If, now, you return to the daylight, which before 

 was perfectly comfortable, it will appear so dazzling that 

 you will perhaps have to close the eyes, and only be 

 able to gaze round with a painful glare. You see 

 thus that we are concerned here not with minute, but 

 with colossal, differences. How now is it possible that, 

 under such circumstances, we can imagine there is any 

 similarity between the picture and reality ? 



Our discussion of what we did not see at first, but 

 could afterwards see in the vault, points to the most 

 important element in the solution ; it is the varying 

 extent to which our senses are deadened by light ; a 



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