40 THE SIGNS OF LIFE [LECT. 11. 



the other optic nerve, or by luminous stimulation of any part of 

 the skin. I think we are bound to accept as correct the results 

 vouched for by an observer of Engelmann's experience, yet, I 

 must confess, that I cannot defend myself from a lingering 

 doubt when I remember how sensitive the retina may be to 

 the weakest trace of light (I have seen distinct reaction to 

 a flash of moonlight lasting y^o-th second, and quite recently, by 

 courtesy of Sir W. Crookes, to the luminosity of radium), and 

 when I consider how difficult it must be to absolutely protect 

 an eye from all trace of direct illumination, while sunlight is 

 reflected on to the other eye or on to the skin. 



REFERENCES 



HOLMGREN. "Ueber die Retinastrome, Cbt. f. d. med. Wissensch." 1871, 



P- 423- 

 DEWAR AND MACKENDRICK. "On the Physiological Action of Light," 



Trans. Roy. Soc., Edinburgh, 1873, p. 141; and, Journ. of Anat. and 



Physiol., vii., 1873, p. 275. 

 KUHNE AND STEINER. I. " Ueber das Electromotorische Verhalten der 



Netzhaut," II. "Ueber Electrische Vorgange," Unters. a. d. Physiol. Inst. 



Heidelberg, iii. and iv., 1880-1881. 

 BECK. "Ueber die bei Belichtung der Netzhaut von Elodone moschata 



entstehenden Actionstrome," Pfliiger's Archiv. Ixxviii., 1899, p. 129. 

 FUCHS. " Untersuchungen u'ber die im Gefolgeder Belichtung auftretenden 



galvanischen Vorgange in der Netzhaut und ihren zeitlichen Ver- 



lauf," P finger's Archiv., Ivi., 1894, p. 408. 

 WALLER. " On the Retinal Currents of the Frog's Eye, excited by Light, and 



excited Electrically," Phil. Trans. Roy. Soc., B vol. 193, 1900, p. 123. 

 WALLER. "On the Blaze-Currents of the Frog's Eyeball," Phil. Trans. 



Roy. Soc., B vol. 194, 1901, p. 183. 

 WALLER. "The Eyeball as an Electrical Organ," Proc. Physiol. Soc., 



10 November 1900. 



ENGELMANN AND GRIJNS. Helmholtz* Festschrift, 1891. 

 GOTCH. "The Time-Relations of the Photo-Electric Changes of the Eye- 

 ball of the Frog," Journ. of Physiol., vol. xxix., 1903, p. 388. 



