POPULAR SCIENTIFIC LECTUKES. 



611 



sphere 195 millions of miles in diameter. Its 

 density r.ncl illuminating power is here only 

 the one forty-thousandth of that with which 

 it left the sun's surface ; and Lambert's 

 ember leads to the conclusion that even the 

 brightest white surface on which the sun's 

 rays fall vertically, has only the one hundred- 

 thousandth part of the brightness of the 

 sun's disk. The moon, however, is a gray 

 body, whose mean brightness is only about 

 one fifth of that of the purest white. 



And when the moon irradiates a body of 

 the purest white on the earth, its brightness 

 is only the hundred-thousandth part of the 

 brightness of the moon itself ; hence the 

 sun's disk is 80,000 million times brighter 

 than a white which is irradiated by the full 

 raoon. 



Now pictures which hang in a room are 

 not lighted by the direct light of the sun, 

 but by that which is reflected from the sky 

 and clouds. I do not know of any direct 

 measurements of the ordinary brightness of 

 the light in a picture gallery, but estimates 

 may be made from known data. With 

 strong upper light and bright light from the 

 clouds, the brightest white on a picture has 

 probably l-20th of the brightness of white 

 directly lighted by the sun ; it will generally 

 be only l-40th, or even less. 



Hence the painter of the desert, even if he 

 gives up the representation of the sun's 

 disk, which is always very imperf *-., will 

 have to represent the glaringly lighted gar- 

 ments of his Bedouins with a white which, 

 in the most favorable case, shows only the 

 l-20th part of the brightness which corre- 

 sponds to actual fact. If he could bring it, 

 with its lighting unchanged, into the desert 

 near the white there, it would seem like a 

 dark gray. I found in fact, by an experi- 

 ment, that lampblack, lighted by the sun, is 

 not less than half as bright as Bhaded white 

 in the brighter part of a room. 



On the picture of the moon, the same white 

 which has been used for depicting the Be- 

 douins' garments must be used for repre- 

 senting the moon's disk, and its reflection 

 in the water ; although the real moon has 

 only one fifth of this brightness, and its re- 

 flection in v.'dter still less. Hence white 

 garments in moonlight, or marble surfaces, 

 even when the artist gives them a gray 

 shade, will always be ten to twenty times as 

 bright in his picture as they are in reality. 



On the other hand, the darkest black 

 which the artist could apply would be 

 scarcely sufficient to represent the real illumi- 

 nation of a white object on which the moon 

 shone. For even the deadest black coatings 

 of lampblack, black velvet, when power- 

 fully lighted appear gray, as we often enough 

 know to our cost, when we wish to shut off 

 superfluous light. I investigated a coating 

 of lampblack, and found its brightness to 

 be about one hundredth that of white paper. 

 The brightest colors of a painter are only 

 about one hundred times as bright as his 

 darkest shades. 



The statements I have mado may perhaps 



appear exaggerated. But they depend upon 

 measurements, and you can control them by 

 well-known observations. According to Wol- 

 laston, the light of the full moon is equal to 

 that of a candle burning at a distance of 12 

 feet. You know that we cannot read by the 

 light of the full mocn, though wej ian read at 

 a distance of three or four feet frol a candle. 

 Now assume that yon suddenly r ssed from 

 a room in daylight to a vault per ctly daik, 

 with the exception of the lighf :>f a single 

 candle. Yoti would at first think ou were in 

 absolute darkness, and at most you would 

 only recognize the candle itself. In any case, 

 you would not recognize the slightest traco 

 of any objects at a distance of 12 feet from 

 the candle. These, however, rre the objects 

 whose ilhirnination is the same as that which 

 the moonlight gives. You would only bo- 

 come accustomed to the darkness after some 

 time, and you would then find your way 

 about without difficulty. 



If, now, you return to the daylight, v/hich 

 before v/as perfectly comfortable, it will ap- 

 pear so dazzling that you will perhaps liavo 

 to close the eyes, and only bo able to gaxo 

 round with a painful glare. You see thua 

 that we are concerned here not with minute, 

 but with colossal, differences. How now is it 

 possible that, under such circumstances, we 

 can imagine there is any similarity between 

 the picture and reality ? 



Our discussion of what we did not see at 

 first, but cou'.d afterward see in the vault, 

 points to the most important element in the. 

 solution ; it is the varying extent to which 

 our senses aro deadened by light ; a process 

 to which we can attach the same name, fa- 

 tigue, as that for the corresponding one in 

 the muscle. Any activity of our nervous sys- 

 tem diminishes its power for the time being. 

 The muscle is tired by work, the brain in 

 lired by thinking, and by mental opera- 

 tions ; the eye is tired by light, and the 

 more so the more powerful the light. Fa- 

 tigue makes it dull and insensitive to new im- 

 pressions, so that it appreciates strong ones 

 only moderately, and weak ones not at all. 



But now you see how different is the aim 

 of the artist when these circumstances art 

 taken into account. The eye of the truvellei 

 in the desert, who is looking at the caravan, 

 has been dulled to the last degree by the 

 dazzling sunshine ; while that of the wan- 

 derer by moonlight has been raised to the 

 extreme of sensitiveness. The condition of 

 one who is looking at a picture differs from 

 both the above cases by possessing ^ certain 

 mean degree of sensitiveness. Accordingly, 

 the painter must endeavor to produce by hi* 

 colors, 011 the moderately sensitive eye of the 

 spectator, the same impression as that which 

 the desert, on the ono hand, produces on the 

 >.-4.aoaea, and the moonlight, on the otuei 

 hand, creates on the untired eye of its ob- 

 server. Hence, along with the actual lu- 

 minous phenomena of the outer world, the 

 different physiological conditions of the ey 

 play a most important part in the work of 

 the artist. What he has to give is not a rnerti 



