f,12 



SM:::;TI: 



o of the object, but a translation of 

 his impression into another scale of sensi- 

 tivenes.s, which belongs to a different degree 

 of impressibility of the observing eye, in 

 which the organ speaks a very different dia- 

 lect in responding to the impressions of the 

 onter world. 



In order to understand to what conclu- 

 nious this leads, I must lirst of all ex- 

 plain the law which Fechner discovered for 

 the scale of sensitiveness of the eye, which 

 is a particular case of the more general 

 psycho-physical la to of the relations of the va- 

 rious sensuous impressions to the irritations 

 which produce them. This law may be ex- 

 pressed as follows : Within very wide limits of 

 iriyhtness, differences in the strength of liyht are 

 qu/itty distinct or appear equal in sensation, if 

 fay form an equal fraction of the total quantity 

 (f liyht compared. Thus, for instance, differ- 

 jnces in intensity of one hundredth of the 

 lolal amount can be recognized without 

 great trouble with very different strengths 

 of light, without exhibiting material differ- 

 ences in the certainty and facility of the os- 

 fcimate, whether the brightest daylight or the 

 light of a good candle be used. 



The easiest method of producing accurately 

 measurable differences in the brightness of 

 two white surfaces, depends on the \ise of 

 rapidly rotating disks. If a disk, like the 

 adjacent one in Fig. 1, is made to rotate very 

 rapidly (that is, 20 to 30 times in a second), 

 it appears to the eye to be covered with three 

 gray rings, as in Fig. 2. The reader must 

 aowever, iigure to himself the gray of these 

 rings, as it appears on the rotating disk of 

 Fig 1, as a scarcely perceptible shade of the 

 ground. When the rotation is r jud each 



rmg of the disk appears Illuminated, as if 

 ill the light which fell itpon it had been uni- 

 formly distributed over its entire surface. 

 Those rings in which are the black bands, 

 hv e somewhat loss light than the quite 

 vr.i.ie ones, nnd if the breadth of the marks 

 is compared with the length of half the cir- 

 cumference of the corresponding ring, wo got 

 the fraction by which the intensity of the 

 light in the white ground of the disk is dimin- 

 ished in the ring in question. If the bands 

 are all equally broad, as in Fig. 1, the inner 

 rings appear darker than the outer ones, for 

 in this latter case the same loss of light is 

 distributed over a larger area than in the 

 former. In this way extremely delicat* 

 shades of brightness may bo obtained, and 

 by this method, when the strength of the il- 

 lumination varies, the brightness always di- 

 minishes by the same proportion of its total 

 value. Now it is found, in accordance with 

 Fechner' s law, that the distinctness of the 

 rings is nearly constant for very different 

 strengths of light. We exclude, of course, 

 the cases of too dazzling or of too dim a 

 light. In both cases the finer distinction* 

 can no longer be perceived by the aye. 



The case is quite different when for diffe 

 ent strengths of illumination we produce 

 differences which always co-respond to the 

 same quantity of light. If, for instance, we 

 close the shutter of a room at daytime, so 

 that it is quite dark, and now light it by a 

 candle, we can discriminate without diffi- 

 cult}' the shadows, such as that of the hand. 

 thrown by the candle on a sheet of whitu 

 paper. If, however, the shutters are again 

 opened, so that daylight enters the room, for 

 the same position of the hand we can no 

 longer recognize the shadow, although ther 

 falls on that part of the white sheet, which ia 

 not struck by this shadow, the same excesn 

 of candle-light as upon the pails shaded by 

 the hand. But this small quantity of light 

 disappears in comparison with the newly 

 ftdded daylight, provided that this strikes all 

 parts of the white sheet uniformly. You set 

 then that, while the difference between can- 

 dle-light and darkness can be easily per- 

 ceived, the equally great difference between 

 daylight, on the one hand, and daylight plu 

 candle-light on the other, can be no longer 

 recognized. 



This law is of gretit importance in discrim- 

 inating between various degrees ci' bright- 

 ness of natural objects. A. white body appear! 

 white because it reflects a large traction, and 

 a gray body appears gray because it reflects a 

 small fraction, of incident light. For differ- 

 ent intensities of illumination, the difference 

 of brightness between the two will always 

 correspond to the same fraction of their total 

 brightness, and hence will be equally percep- 

 tible to our eyes, provided wo do not ap- 

 proach too near to the upper or the lowei 

 limit of the brightness, for which Fechner' .v 

 law no longer holds. Hence, on the whole 

 the painter can produce what appears an 

 equal difference for the spectator of his pio- 

 ture, notwithstanding the varying strength oJ: 



