48 ox GOETHE'S SCIENTIFIC RESEARCHES. 



any of its constituents). The direct mixture of dark and light, 

 of black and white, gives grey ; the colours must therefore owe 

 their existence to some form of the co-operation of light and 

 shade. Goethe imagines he has discovered it in the phenomena 

 presented by slightly opaque or hazy media. Such media usually 

 look blue when the light falls on them and they are seen in 

 front of a dark object, but yellow when a bright object is looked 

 at through them. Thus in the daytime the air looks blue 

 against the dark background of the sky, and the sun, when 

 viewed, as is the case at sunset, through a thick and hazy 

 stratum of air, appears yellow. The physical explanation of 

 this phenomenon, which, however, is not exhibited by all such 

 media, as, for instance, by plates of unpolished glass, would lead 

 us too far from the subject. According to Goethe, the semi-opaque 

 medium imparts to the light something corporeal, something of 

 the nature of shade, such as is requisite, he would say, for the 

 formation of colour. This conception alone is enough to perplex 

 any one who looks upon it as a physical explanation. Does he 

 mean to say that material particles mingle with the light and 

 fly away with it ? But this is Goethe's fundamental experiment, 

 this is the typical phenomenon under which he tries to reduce 

 all the phenomena of colour, especially those connected with 

 the prismatic spectrum. He looks upon all transparent bodies 

 as slightly hazy, and assumes that the prism imparts to the 

 image which it shows to an observer something of its own 

 opacity. Here, again, it is hard to get a definite conception of 

 what is meant. Goethe seems to have thought that a prism 

 never gives perfectly defined images, but only indistinct, half- 

 obliterated ones, for he puts them all in the same class with the 

 double images which are exhibited by parallel plates of glass 

 and by Iceland spar. The images formed by a prism are, it 

 is true, indistinct in compound light, but they are perfectly 

 defined when simple light is used. If you examine, he says, a 

 bright siirface on a dark ground through a prism, the image is 

 displaced and blurred by the prism. The anterior edge is 

 pushed forward over the dark background, and consequently a 

 hazy light on a dark ground appears blue, while the other edge 



