OF ARTS AND SCIENCES. 409 



Volkmann, Goethe, Brewster, and many others, have claimed to be 

 able to distinguish with one motionless eye two component parts of a 

 mixed color. Hehnholtz urges that this is always a matter of judgment, 

 and not of sensation. We are used, he says, to seeing things in the 

 changing lights and shadows of morning, noon, and night, and by arti- 

 ficial illumination ; and so unconsciously acquire the ability to judge 

 what is due to the medium, and what comes from the object ; and thus 

 can often do so correctly the first time, when tliin, while paper is spread 

 over a colored surface. Wundt urges against Zenker's ingenious the- 

 ory, that we ought to see white light as mixed. Now, in the first place, 

 it is by no means proven that the eye cannot be trained to see one 

 color through another at a distance from it, — which is the most favor- 

 able form of mixing; but even here each shade of a composite color 

 may have a different local sign, while, as in our hypothesis, the elastic 

 mechanical action of disks on each other — lying, as they do, so close 

 together that no microscope can distinguish them in a fresh retina 

 — prevents distinct perception. Investigation is much needed here. 

 Meanwhile, we must not forget that the retina is not the only surface 

 where different modes of irritation coalesce in a single sensation, and 

 that the difficulty may be solely due to inadequate cultivation of atten- 

 tion and discrimination. 



Our theoi-y thus involves a redistribution of the causes of many 

 well-known phenomena of vision. We must distinguish, first, those 

 due to direct action of light on sensitive disks ; second, those due to 

 the action of these disks on each other; and, last, those due to the neu- 

 ral action thus occasioned : and the problem iiow before us is to deter- 

 mine how far such phenomena as after-images, contrast, persistence 

 of impression, &c., are due to each of these influences. Such familiar 

 facts as that change of brightness causes change of hue ; that pale and 

 dark colors are more contrasted than those tiiat are full ; that greater 

 intervals intensify and emphasize colors, while small differences in tone 

 seem most contrasted, — would indicate that contrast is not, as has been 

 lately claimed, all a matter of subjective judgment, but that it may be 

 due, in part, to the second of these causes. Complementary after- 

 images, it is quite certain, cannot be explained by any theory of musi- 

 cal harmonics; for the musical fifih would give us a greater, the third 

 a less, than the complementary interval on the color scale. We should 

 expect, if the disks respond by vibration, that their motion would be 

 far less rapid than that of light waves, corresponding perhaps as a very 

 small multiple, or as a lower harmonic or difference tone; or, again, 

 there may be one curve for the elasticity of disk action, or of the 



