8 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



disappeared, leaving nothing but the uniform background and the 

 fixation-object. Similar experiments with papers of various colors 

 showed that certain tones disappear more rapidly in indirect vision than 

 do others. The degree of persistence of the retinal image is, in his 

 opinion, conditioned by two factors the color of its object and the 

 angular distance of the object from the fixation-point. The latter factor 

 he explains, in part at least, in terms of luminosity of image ; the former 

 he makes no attempt to explain, nor even to describe in satisfactory 

 detail. It is to be noticed in this connection that the manner of disap- 

 pearing, as described by Troxler, is peculiar and interesting. The colors 

 gradually paled out and vanished, frequently reappearing " floating 

 into view as out of still water " before ultimately disappearing.* In 

 no case do they seem to have been followed by an after-image, f 



Purkinje t confirmed Troxler's observations as to the rapid fading 

 of color from peripheral images and endeavored to bring them into 

 relation with certain subjective phenomena the disappearance and 

 reappearance of objects which occurs when one is drowsy, and the float- 

 ing phantoms which may be observed when one fixates a point in a dark 

 room both of which classes of phenomena he describes at length in his 

 earlier paper. In his later paper he gives a more detailed discussion of 

 central and peripheral vision, for which he proposes the names, now 

 commonly employed, of direct and indirect vision. 



There is, he says, but a single retinal point the fovea at which 

 the brightness, the color, and the form of objects are seen with maximal 

 clearness. This three-fold sensitivity of the retina diminishes with in- 



*This reappearance of objects which had previously disappeared from view 

 is doubtless due to eye-movements. Failure to maintain a constant fixation 

 would result in the exposure, to the action of the stimulus, of a non-fatigued area 

 of the retina; and there would ensue a series of appearances and disappearances 

 correlate with the successive shift-ings of regard and the subsequent fatigue of 

 the new area stimulated. The absence of after-images upon the peripheral retina 

 has been pointed out by several investigators; it was also characteristic of our 

 own experiments. (See pp. 56 and 58.) 



tit is also to be noted that Aubert refers to Troxler as having been the first 

 to discover that objects appear colorless in indirect vision (Grundziige der 

 Physiologischen Optik, S. 539). We can find no mention of this phenomenon in 

 Troxler's paper. It is true that he speaks of the colored papers merging into the 

 uniform (blue) background; but that this is a final stage of the process, Troxler 

 himself is careful to emphasize. What Troxler is interested in demonstrating is 

 the relatively rapid chromatic adaptation of the peripheral retina. That under 

 certain conditions it is wholly insensitive to color, clearly escaped his notice. 



t Johannes Evangelista Purkinje. Beobachtungen und Versuche zur Physi- 

 ologic der Sinne, I, Beitrage zur Kenntnis des Sehens in subjectiver Hinsicht, 

 Prag, 1823, Sections V and IX, particularly S. 77; H, Neu-e Beitrage, u. s. w., 

 Berlin, 1825, S. 3-25. Various spellings of the name of this author have crept 

 into the literature. We have adopted the form given 'by Purkinje himself in the 

 two papers cited. 



