HarUhorne.J Ji^^ [April 21, 



its light weight, for foreign correspondence, be laid over a piece of brightly 

 colored material (paper, silk, tlannel, or any other) and then on the latter 

 a small black or dull colored strip or fragment of any kind be laid, under 

 the j)ai)er, i\\'\9, fratjment or strip will ai)pear to be of tlie color complemen- 

 tary to that of tlic larger and brighter ground upon which it is placed. 

 Any color will answ^er for this experiment; although the effects are most 

 distinct with red and green grounds; tolerably so with bright yellow; least 

 so witli blue, purple, and all dark sliades. It makes no diHerence what is the 

 ?iue of t\w fragment or stri]) upon the brighter ground under the paper, so 

 long as it is duller or darker, as well as considerably smaller, than the ground 

 on which it is laid; it will always have the color complementary to that 

 ground. Put, for example, together, under tlu; thin paper, on a bright 

 green ground, strips of blue, red and yellow glass, and one also of black 

 worsted or paper; all of these, seen through the paper, will be, alike, dis- 

 tinctly red. 



Now, on observing these facts, let it be noticed, especially, that the effect 

 produced, the complementary color given, is iristantaneous. Tiie moment 

 you look at the bright ground with the dull or indifferent strip over it, you 

 at once see that strip with the complementary color. Lift the paper, and 

 the real hue, or blackness, is seen; replace it, and it becomes again changed 

 as before. There is here no time for fatigue of the retina, or of any part of it. 

 Again, if the bright ground have, around it, a margin of the paper which 

 is laid over it, that margin will have a fiiint but distinct flush of the com- 

 plementary color. If the bright ground be moved to and fro, this over- 

 flush of color may be, sometimes beautifully, shown, following the line or 

 edge of the moving mass of strong color; and most distinct near the edge. 

 This effect, also, is instantaneous; there is no opportunity for fatigue in any 

 way to explain it. 



Finding then, this " fatigue" theory quite insufficient for these facts,atten- 

 tion to the phenomena of color-shadows above referred to, has impressed 

 me with a belief that the same theory falls short -with them also. They 

 are, likewise, immediate, momentary effects. You look at the white 

 ground, bathed in colored light, which light is partly intercepted by a nar- 

 row opaque object; and at once you see the shadow of the latter, having the 

 complementary color; there is no possible time for fatigue, or for loss of any 

 portion of the retinal sensibility. 



Consideration may next be in place, uponaremarkofllelmholtz,* which, 

 though only parenthetically made by him, has very considerable importance. 

 Eriglit objects, as flames, or the sun, Avhen looked at, give subsequent spec- 

 tral eflects unlike those described as negative. They are, indeed, for a time 

 at.least, positive spectra. Ilelmholtz speaks, moreover, of from half a minute 

 to five minutes' contemplation of a bright or colored object to produce the 

 ordinary negative or comi)lementary color spectrum by looking upon a 

 white ground. I have found from five to ten seconds ample, with a kero- 

 sene students' lamp; and less time still, with strong sunlight. 



♦Popular Lectures, transl., 1873. 



