376 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



of visual purple. This view is certainly incorrect. A bleached 

 retina is more strongly fluorescent than one in which the visual 

 purple is unreduced (Kiihne 27 ), although the bleached substance 

 itself may possess fluorescent properties, since Nagel and 

 Himstedt ( 28 ) observed that a bleached solution of visual purple 

 was more strongly fluorescent than the solvent alone. 



If then we accept, as a working hypothesis, the view that the 

 rods and visual purple form a link in the chain of processes by 

 means of which certain forms of stimuli are, under particular 

 conditions, associated with sensations, we must not, in the present 

 state of the question, attempt to assign to them any precise 

 physical or physiological share in the process. 



In conclusion, the reader need not suppose that the cones of 

 the periphery are functionless, that in broad daylight peripheral 

 vision is at all of the type we have been studying. As a matter of 

 fact, at the extreme periphery which is normally quite colour-blind, 

 brightness values are altogether different from those of the " dark " 

 eye, as is shown in the following table (v. Kries, 17 p. 199) : 



Na line = 100 



Wave length . . 680 651 629 608 589 573 558 530 513 

 Peripheral value 



daylight . . 9'6 37'5 77'5 101 100 79'6 522 28-5 14-6 

 Peripheral value 



twilight . . ? 3-4 140 35'5 100 256 351 321 198 



The study of this problem of visual adaptation illustrates well 

 the patient and laborious experimental work necessary to de- 

 monstrate even a limited range of phenomena, the complexity of 

 results which appear at first simple and the necessity of caution 

 in framing satisfactory hypotheses. It has been chosen to de- 

 scribe because it is so instructive from these points of view. 



BIBLIOGRAPHY 



(This list is not, of course, complete. The papers marked with an asterisk 

 contain extensive bibliographies.) 



1 Charpentier, Arch. d'Opthalmologie, vol. iv., pp. 291-323. 



2 A. Tschermak, Pfliiger's Arch., vol. 70, pp. 297-328. A critical summary 

 of work up to 1902 will be found in * Die Helldunkeladaptation des Auges 

 und die Funktion der Stabchen und Zapfen, by A. Tschermak, Ergebnisse 

 der Physiologic, 1st Jahrgang, 2nd part, pp. 695, &c. 



