ON THE RELATION OF OPTICS TO PAINTING. 103 



"brightness may be obtained, and by this method, when 

 the strength of the illumination varies, the brightness 

 always diminishes by the same proportion of its total 

 value. Now it is found, in accordance with Fechner's 

 law, that the distinctness of the rings is nearly con- 

 stant for very different strengths of light. We ex- 

 clude, of course, the cases of too dazzling or of too dim 

 a light. In both cases the finer distinctions can no 

 longer be perceived by the eye. 



The case is quite different when for different 

 strengths of illumination we produce differences which 

 always correspond to the same quantity of light. If, 

 for instance, we close the shutter of a room at daytime, 

 so that it is quite dark, and now light it by a candle, 

 we can discriminate without difficulty the shadows, such 

 as that of the hand, thrown by the candle on a sheet 

 of white paper. If, however, the shutters are again 

 opened, so that daylight enters the room, for the same 

 position of the hand we can no longer recognise the sha- 

 dow, although there falls on that part of the white sheet, 

 which is not struck by this shadow, the same excess 

 of candle-light as upon the parts shaded by the hand. 

 But this small quantity of light disappears in compari- 

 son with the newly added daylight, provided that this 

 strikes all parts of the white sheet uniformly. You 

 see then that, while the difference between candle-light 

 and darkness can be easily perceived, the equally great 



