106 ON THE EELATION OF OPTICS TO PAINTING. 



brightness ; while, for a brightness which is too high 

 <or too low, appreciable divergences are met with. 



At both extremes of luminous intensity the eye is 

 less sensitive for differences in light than is required by 

 that law. With a very strong light it is dazzled ; that 

 is, its internal activity cannot keep pace with the ex- 

 ternal excitations ; the nerves are too soon tired. Very 

 bright objects appear almost always to be equally 

 bright, even when there are, in fact, material differ- 

 ences in their luminous intensity. The light at the 

 edge of the sun is only about half as bright as that at 

 the centre, yet none of you will have noticed that, if 

 you have not looked through coloured glasses, which 

 reduce the brightness to a convenient extent. With 

 a weak light the eye is also less sensitive, but from the 

 opposite reason. If a body is so feebly illuminated 

 that we scarcely perceive it, we shall not be able to 

 perceive that its brightness is lessened by a shadow 

 by the one hundredth or even by a tenth. 



It follows from this, that, with moderate illumina- 

 tion, darker objects become more like the darkest 

 objects, while w^ith greater illumination brighter ob- 

 jects become more like the brightest than should be 

 the case in accordance with Fechner's law, which 

 holds for mean degrees of illumination. From this 

 results, what, for painting, is an extremely characteristic 



