220 THE POPULAR SCIENCE MONTHLY. 



this coloring of the sky was due to the shining of the sun upon a tur- 

 bid medium with darkness behind. He by no means understood the 

 physical action of turbid media, but he made a great variety of experi- 

 ments bearing upon this point. Water, for example, rendered turbid 

 by varnish, soap, or milk, and having a black ground behind it, always 

 , appeared blue when shone upon by white light. When, instead of a 

 black background, a bright one was placed behind, so that the light 

 shone, not on, but through the turbid liquid, the blue color disappeared, 

 and he had yellow in its place. Such experiments are capable of end- 

 less variation. To this class of effects belongs the painter's " chill." 

 A cold, bluish bloom, like that of a plum, is sometimes observed to cov- 

 er the browns of a varnished picture. This is due to a want of optical 

 continuity in the varnish. Instead of being a coherent layer it is broken 

 up into particles of microscopic smallness, which virtually constitute a 

 turbid medium and send blue light to the eye. 



Goethe himself describes a most amusing illustration, or, to use his 

 own language, " a wonderful phenomenon," due to the temporary action 

 of a turbid medium on a picture : " A portrait of an esteemed theo- 

 logian was painted several years ago by an artist specially skilled in 

 the treatment of colors. The man stood forth in his dignity clad in a 

 beautiful black velvet coat, which attracted the eyes and awakened the 

 admiration of the beholder almost more than the face itself. Through 

 the action of humidity and dust, however, the picture had lost much 

 of its original splendor. It was therefore handed over to a painter 

 to be cleaned and newly varnished. The painter began by carefully 

 passing a wet sponge over the picture. But he had scarcely thus re- 

 moved the coarser dirt, when to his astonishment the black velvet sud- 

 denly changed into a light-blue plush ; the reverend gentleman acquir- 

 ing thereby a very worldly, if, at the same time, an old-fashioned ap- 

 pearance. The painter would not trust himself to wash further. He 

 could by no means see how a bright blue could underlie a dark black, 

 still less that he could have so rapidly washed away a coating capable 

 of converting a blue like that before him into the black of the original 

 painting." 



Goethe inspected the picture, saw the phenomenon, and explained 

 it. To deepen the hue of the velvet coat the painter had covered it 

 with a special varnish, which, by absorbing part of the water passed 

 over it, was converted into a turbid medium, through which the black 

 behind instantly appeared as blue. To the great joy of the painter, 

 he found that a few hours' continuance in a dry place restored the 

 primitive black. By the evaporation of the moisture the optical 

 continuity of the varnish (to which essential point Goethe does not 

 refer) was reestablished, after which it ceased to act as a turbid 

 medium. 



This question of turbid media took entire possession of the poet's 

 mind. It was ever present to his observation. It was illustrated by 



