New Analysis of Solar Light. 411 



A second possible source of error is to be found in the phy- 

 siological effect of contrast, wliicli might easily prejudice the 

 judgement of the colours, particularly when we observe a weakly 

 illuminated space beside one which is brightly illuminated. 

 Briicke* has lately drawn attention to the fact, that even quite 

 obscure portions of the field of view appear, beside bright colours, 

 to have a luminosity poured over them ; that this luminosity is 

 sometimes of the same colour as the light which excites it, some- 

 times complementary to the latter, and sometimes altogether 

 different. He names the colour of this luminosity the induced 

 colour. By the degree of brightness which he made use of, he 

 found that red induced its complementary green, but that green 

 induced green, violet, blue, but that blue and yellow did not 

 induce any decided colour. A repetition of these experiments 

 with different degrees of brightness, convinces me that the ex- 

 pression of Briicke must be modified; when very bright light is 

 made use of, the same colour is always shed over the dark portion 

 of the field, a phaanomenon the possible cause of which has been 

 spoken of above. With weak light the induced colour is always 

 the complementary one, which, as Briicke also has remarked, 

 becomes much more vivid when the eye is moved than when it 

 is fixed upon a point ; with medium light the deportment of 

 different colours is different ; sometimes the same colour, some- 

 times its opposite is produced ; sometimes indefinite colourings, 

 as if the opposed ph?euomena were struggling for pre-eminence. 

 I have also found, in coincidence with the observation of Briicke, 

 that the complementary colour of red is more easily induced 

 than those of violet or green. 



To this source it appears to me must be referred a surprising 

 experiment of Brewster's, by which he sought to demonstrate 

 the presence of green light in yellow, orange, and even in red 

 towards the line C. As absorbing medium he made use of port 

 wine, Peruvian balsam, pitch, sulphur-balsam, or red mica. I 

 have repeated the experiments with Peruvian balsam, sulphur- 

 balsam and pitch. Thin layers of these substances permit the 

 red, yellow, and green of the spectrum to stand, while they 

 extinguish blue and violet. In tliis case, however, green appears 

 to extend as far as the line D, whose real position is in golden- 

 yellow, and frequently reaches even beyond this to the vicinity 

 of the reddish-orange. Tlie green seems to abut immediately 

 against the red. Hence the yellow-green, yellow, golden- yellow, 

 and even the orange tone ap])ear to have become green, and the 

 latter is so vivid that it is indeed difficult to conceive that it 

 could be a subjective illusion. The presence of such an illusion 



* " Untcrsuchung iiber Subjective Farben," Denksch. der Akad. d. WiS' 

 senschaft zu IVien, vol. iii. 



