392 HISTORY OF OPTICS. 



we see through a dim medium ("em triibes mit- 

 tel"). Light in itself is colourless ; but if it be seen 

 through a somewhat dim medium, it appears yellow ; 

 if the dimness of the medium increases, or if its 

 depth be augmented, we see the light gradually 

 assume a yellow-red colour, which finally is height- 

 ened to a ruby-red. On the other hand, if dark- 

 ness is seen through a dim medium which is illumi- 

 nated by a light falling on it, a blue colour is seen, 

 which becomes clearer and paler, the more the 

 dimness of the medium increases, and darker and 

 fuller, as the medium becomes more transparent; 

 and when we come to " the smallest degree of the 

 purest dimness," we see the most perfect violet 17 . 

 In addition to this " doctrine of the dim medium," 

 we have a second principle asserted concerning 

 refraction. In a vast variety of cases, images are 

 accompanied by "accessory images," as when we 

 see bright objects in a looking-glass 18 . Now, when 

 an image is displaced by refraction, the displace- 

 ment is not complete, clear and sharp, but incom- 

 plete, so that there is an accessory image along 

 with the principal one 19 . From these principles, 

 the colours produced by refraction in the image of 

 a bright object on a dark ground, are at once 

 derivable. The accessory image is semitransparent' ; 

 and hence that border of it which is pushed for- 

 wards, is drawn from the dark over the bright, and 



17 Farbenlehre, 150, p. 151. " Ib. 223. 



19 Ib. 227. 20 Ib. 238. 



