S*CT. XIX. ABSORPTION OF LIGHT. 155 



Upon this property of unequal absorption, the colors of 

 transparent media depend. For they also receive their 

 color from their power of stopping or absorbing some of 

 the colors of white light and transmitting others. As 

 for example, black and red inks, though equally homo- 

 geneous, absorb different kinds of rays ; and when ex- 

 posed to the sun, they become heated in different de- 

 grees ; while pure water seems to transmit all rays 

 equally, and is not sensibly heated by the passing light 

 of the sun. The rich dark light transmitted by a smalt- 

 blue finger-glass is not a homogeneous color like the 

 blue or indigo of the spectrum, but is a mixture of all 

 the colors of white light which the glass has not ab- 

 sorbed. The colors absorbed are such as mixed with 

 the blue tint would form white light. When the spec- 

 trum of seven colors is viewed through a thin plate of 

 this glass they are all visible ; and when the plate is 

 very thick, every color is absorbed between the extreme 

 red and the extreme violet, the interval being perfectly 

 black : but if the spectrum be viewed through a certain 

 thickness of the glass intermediate between the two, it 

 will be found that the middle of the red space, the whole 

 of the orange, a great part of the green, a considerable 

 part of the blue, a little of the indigo, and a very little 

 of the violet, vanish, being absorbed by the blue glass : 

 and that the yellow rays -occupy a larger space, cover- 

 ing part of that formerly occupied by the orange on one 

 side, and by the green on the other. So that the blue 

 glass absorbs the red light, which when mixed with the 

 yellow constitutes orange ; and also absorbs the blue 

 light, which when mixed with the yellow forms the 

 part of the green space next to the yellow. Hence by 

 absorption, green light is decomposed into yellow and 

 blue, and orange light into yellow and red. Conse- 

 quently the orange and green rays, though incapable of 

 decomposition by refraction, can be resolved by absorp- 

 tion, and actually consist of two different colors possess- 

 ing the same' degree of refrangibility. Difference of 

 color, therefore, is not a test of difference of refrangi- 

 bility, and the conclusion deduced by Newton is no 

 longer admissible as a general truth. By this analysis 

 of the spectrum, not only with blue glass, but with a 



