ON GOETHE'S SCIENTIFIC RESEARCHES. 49 



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 anterior edge appears in 

 front of the ground, the posterior edge behind it, and not vice 

 versd, 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. Every one 

 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 simply a geometrical point, in which the 

 reflected rays, if produced backwards, would intersect. And, 

 accordingly, no one expects the image to produce any real effect 

 behind the mirror. In the same way the prism shows us images 

 of objects which occupy a different position from the objects 

 themselves ; that is to say, the light which an object sends to 

 the prism is refracted by it, so that it appears to come from an 

 object lying to one side, called the image. This image, again ; 

 is not real ; it is, as in the case of reflection, the geometrical 

 point in which the refracted rays intersect when produced back- 

 wards. And yet, according to Goethe, this image is to produce 

 real effects by its displacement; the displaced patch of light 

 makes, he says, the dark space behind it appear blue, just as an 

 imperfectly transparent body would, and so again the displaced 

 dark patch makes the bright space behind appear reddish-yellow. 

 That Goethe really treats the image as an actual object in the 

 place it appears to occupy is obvious enough, especially as he is 

 compelled to assume, in the course of his explanation, that the 

 blue and red edges of the bright space are respectively before 

 and behind the dark image which, like it, is displaced by the 

 prism. He does, in fact, remain loyal to the appearance pre- 

 sented to the senses, and treats a geometrical locus as if it were 

 a material object. Again, he does not scruple at one time to 

 make red and blue destroy each other, as, for example, in the 



I. E 



