5 8o THE POPULAR SCIENCE MONTHLY. 



would. This is, indeed, to the painter, a precious resource, for repre- 

 senting, in his pictures, high degrees of luminosity, and is often em- 

 ployed with most happy effect. According to the careful experiments 

 by Aubert, white paper is only fifty-seven times lighter than black 

 paper, and the painter is in the predicament of being obliged to repre- 

 sent the vast range of natural illumination within these very narrow 

 limits ; hence the desirability of employing an artifice of this kind to 

 overcome a difficulty which, if fairly met, would prove insuperable. 



The considerations that I have just presented explain to us, quite 

 readily, the curious circumstance that light of any color, if very bright, 

 is at last accepted by the eye for white, all three sets of nerves finally 

 reaching, in the order indicated, their point of maximum stimulation. 

 You can repeat for yourselves a simple experiment of Helmholtz's, in 

 this connection : hold before the eyes, for some little time, a plate of 

 stained glass ; the color may be red, yellow, blue, or green ; after a 

 while you will come to consider the brightest objects in your field of 

 view white ; as, for example, a gas-flame, the sky, or white paper. 

 In point of fact, to be quite frank, white is only a relative sensation, 

 and, if any thing like equality of stimulation is produced in the three 

 sets of nerves, we finally accept the tint for white. I have especially 

 arranged an experiment to illustrate this point : We have now upon the 

 screen two large squares of light ; one is deep red, the other green : I 

 remove from the lantern a large plate of green glass ; the red square 

 has retained its color, and is now brighter, but the other square has 

 become white or almost white. On removing the red glass, the red 

 square on the screen is replaced by a white one, and we now for the 

 first time see that its companion, which a moment ago we were ready 

 to take for white, has a decidedly green hue; in fact, all the while the 

 light producing it has been passing through a plate of pale-green glass, 

 which was behind the others. Let me take away this plate, and now 

 at last we have both our squares illuminated with pure white light. Is 

 this light really white ? Not at all ; it has been tinged decidedly yel- 

 low, by passing through a pale-yellow glass, which has been concealed 

 in the apparatus all the time as a reserve, and, on removing this glass, 

 we find that the light we were ready to accept for white looks yellow, 

 when compared with the purer light of the lantern. Finally, if we 

 could throw a sample of daylight on the screen, we should again see 

 that the light of the lantern itself is not white, but yellowish. White 

 is evidently only a relative sensation. 



In some of the preceding experiments it has been seen that, as we 

 increase the actual brightness of any colored light, red for example, so 

 does the sensation produced also increase, but usually at a slower rate. 

 Now, it happens that some of the sensations increase more rapidly 

 than others ; for example, the sensation for red or yellow increases 



