56 ON Goethe's scientific eesearches. 



it ? But this is Groethe's fundamental experiment, this 

 is the typical phenomenon under which he tries to reduce 

 all the phenomena of colour, especially tliose connected 

 with the prismatic spectrum. He looks upon all trans- 

 parent 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 per- 

 fectly 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 surface 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 is covered 

 by the image of the black surface which comes after it, 

 and, consequently, being a light image behind a hazy 

 dark colour, appears yellowish-red. But why the an- 

 terior edge appears in front of the ground, the posterior 

 edge behind it, and not vice versa, he does not explain. 

 Let us analyse this explanation, and try to grasp clearly 

 the conception of an optical image. When I see a 

 bright object reflected in a mirror, the reason is that 

 the light which proceeds from it is thrown back exactly 

 as if it came from an object of the same kind behind the 

 mirror. The eye of the observer receives the impression 

 accordingly, and therefore he imagines he really sees the 

 object. Everyone knows there is nothing real behind 

 the mirror to correspond to the image — that no light can 

 penetrate thither, but that what is called the image is 



