MODERN OPTICS AND PAINTING. 



579 



to the preceding, though, at first sight, the connection is not very- 

 evident. Any color, if very luminous, seems paler than it really 

 is. This simple piece of apparatus, where a bat-wing gas-flame is 

 placed between a sheet of card-board and a plate of stained glass, 

 will serve for experimental demonstration. The glass is red, and the 

 paper seen through it appears of a deep-red hue, but the gas-flame 

 itself, being much more luminous than the paper, does not look red at 

 all ; its tint is orange (Fig. 9). Replacing the red glass by green, 



Ftg. 9. 



we have the paper appearing with a deep-green hue, while the flame 

 seems greenish-yellow. Let us see if we can explain these curious 

 changes of tint by Young's theory. The red glass used in the first 

 experiment transmits to the eye only red light, or light capable of 

 stimulating mainly the red nerves ; but, if we increase its intensity 

 beyond a certain point, its action on the red nerves begins to flag, 

 and we soon have a state of things where a further increase of the 

 red light produces no effect at all on the red nerves, they being 

 already stimulated up to the maximum point. But, according to our 

 theory, this red light has all along been acting, to some extent, on 

 the green, and to a less extent on the violet nerves ; and, as we add 

 to its intensity, it acts still more powerfully on them, so that espe- 

 cially the green nerves come more and more into play, and a green is 

 added to the original red sensations ; the result, of course, is the sen- 

 sation of orange. 



The explanation of the tint obtained in the other experiment is 

 quite similar. The green nerves are first stimulated up to their maxi- 

 mum point by green light of a certain strength, a further increase 

 of its intensity brings into play the other two sets of nerves, particu- 

 larly the red, and the tint quite naturally becomes greenish-yellow. 

 You remember that, in a previous experiment, we found that a mix- 

 ture of much green with a little red light gave a greenish-yellow. 

 The nerves for violet light always lag behind the others, as will after- 

 ward be shown by a particular experiment. 



The general effect, then, of a very bright illumination on natural 

 objects is to cause their colors to appear paler than they otherwise 



