OF ARTS AND SCIENCES. 405 



paper. No noticeable paling of red or of violet could be observed. 

 The experiment was afterward varied by getting a posiiive after-image 

 of the whole of a short spectrum at once, the middle of which still 

 seemed nearly or quite white. Thinking that this might be dne to 

 the greater intensity which spectra formed by most prisms seem to 

 liave to the normal eye near the middle, these spectra were thrown 

 upon red and violet paper, which absorbed most of the green rays ; but 

 still the effect was the same. 



Now, if we suppose a series of sensitive elements — let us say disks, 

 like the rattles on the tail of a snake — at the ends of the coni:s, each 

 responding by sympathetic vibrations, or otherwise, to the action of 

 waves in the ligiit-ether, of corresponding length, the perception of 

 white light woidd re(iuire the simultaneous agitation of all, or at least 

 of several, groups of these disks. Let us assume also, for the present, 

 that these disks are arranged in a spectral order, — those sensitive to 

 red near the point, those sensitive to violet near the base, of the cone, 

 — each disk being transparent to all waves of ^'reater length than those 

 to which it is best fitted to respond. If this were the case, agitation 

 of a group of disks near the middle of what we will call the cone- 

 spectrum might mechanically agitate the groups on either side, and 

 thus give rise to a wave of disturbance, which, passing to both ends 

 of the series, would cause a sensation of white, which the agitation 

 of either end would not do to any such extent. 



Again, it is well known that pressure, either mechanically applied, or 

 caused l)y retinal congestion, often causes pure colored as well as white 

 iinagrs. This has never been satisfactorily explained on the hypothesis 

 of three sets of cones, or rods. So far as the effects of pressure have 

 been observed on retinal purple of fresh ej'es, the effect is always the 

 same. If we assume that increasing degrees of pressure excite waves 

 of disturbance of increasing length, involving a larger number of disks, 

 we can readily believe that the effects of fatigue, determining the de- 

 gree of instability of different segments along the cone-spectrum, would 

 account for tiie various color-sensations thus produced. 



Instead of three species of terminal organs, the modern form of 

 Young's hypothesis assumes three sets of sensitive fibres, responding 

 to the irritations of the three ground colors. The facts of red blind- 

 ness afford, perhaps, the strongest ground for this theory. According 

 to our hypothesis, however, red is perceived in the outer plane of the 

 retina at the end of the cones. If we consider the delicacy and com- 

 paratively exposed position of these red disks among the coarser pig- 

 ment cells of the choroid, and especially if we admit it to be proven 



