IO COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



Although Purkinje admits that the problem baffles him, he makes 

 an important contribution toward its final solution. He was the first to 

 observe that sensitivity to light has a wider retinal extension than sen- 

 sitivity to color, that the different colors have retinal zones of different 

 extension, and that colored objects appear in different color tones at 

 different parts of the retina. He also discovered the significance of 

 retinal adaptation in color vision; for in the discussion of his color 

 experiments he recommends that the eye be closed for a time after each 

 exposure, " uni seine Empfindlichkeit zu sammeln " (1. c., S. 6). 



Szokalsky* also found that sensitivity to color is not uniformly dis- 

 tributed over the whole retina, and that colored objects appear in differ- 

 ent tones when their images move across the retina. His apparatus 

 consisted of a black background, containing at its center a white fixation- 

 point. Pigment colors were moved in from the edges of the back- 

 ground towards the center. His experiments showed that purplish-red 

 appears first black, then blue, then violet, and finally assumes a tone of 

 purple-red, only when it is near the fixation-point ; violet passes through 

 black and blue before becoming violet; bright blue and rose-red appear 

 white at the periphery. "The other colors," he adds, "gave similar 

 results." In consequence of these experiments, Szokalsky conceives the 

 retina to be made up of three concentric zones; the outermost zone is 

 assumed to be sensitive only to black and white, the intermediate zone 

 only to blue, yellow, black, and white, while the innermost zone is 

 capable of sensing these four, and red besides. This happy conjecture 

 was unfortunately not elaborated in detail by its author ; its chief inter- 

 est lies in the fact that it foreshadows the brilliant discovery made half 

 a century later by Bull and Hess. 



Hueckf attacked the problem by means of a new method. He 

 endeavored to determine the minimal extent of retinal image which is 

 capable of producing a sensation of color, at the different parts of the 

 retina, i. e., he set out to determine the minimum visibile of color for 

 various degrees of eccentricity. He, too, found that color sensitivity 

 decreases with increase of distance from the fovea; he confirmed the 

 existence of Purkinje's black-white zone, but does not mention having 

 observed any transitions of color tone. His essential contribution to the 



*V. Szokalsky. Essai sur les sensations des couleurs, etc., Annales d'Oculist, 

 II, 1839, and III, 1840. These papers were reprinted in book form, under the 

 same title, Paris, 1841 ; they were 'Subsequently elaborated and published in Ger- 

 man (Ueber die Empfindungen der Farben in physiologischer und pathologischer 

 Hinsicbt, Giessen, 1842). 



f A. Hueck. Von den Grenzen des Sehenvermogens, 'Miiller's Archiv. f. Anat, 

 Phsyiol. und wiss. Med., 1840, S. 83-98. 



