( 699 ) 



upon the retina of the eye under observation. The rays proceeding 

 from the crater of an arc lamp, which passed through a collimator 

 slit in close proximity were dispersed by a spectroscopic arrange- 

 ment and from the spectrum so obtained any desired portion could 

 be isolated by a simple device. 



If we made use of rays lying between the wave lengths ^:= 0,590 ;t 

 and I = 0,497 [x, whose green central part — about ;. = 0,544 ft — 

 may be considered to have relatively a very strong effect on the 

 eye ^), we could by the aid of suitably chosen diaphragms vary the 

 light intensities in the proportion of 1 to 10'', and with the weakest 

 intensity could obtain galvanometric deflections of several centi- 

 metres. The arrangement of our experiments did not permit of our 

 easily diminishing the light further in an accurately measurable 

 manner, but we hope later to be able to do so. 



In some experiments white light has been used, which of course 

 could be taken stronger than the spectral green. In this case all the 

 rays of the visible spectrum lie at our disposal, and the light may 

 be further increased by widening the slit or by replacing it with 

 the crater itself. According to a rough calculation the intensity of 

 the white light used by us, that is to say of the combined rays 

 lying within the limits of the visible spectrum, is about 10 times 

 greater than our maximum green. The intensities of the weakest 

 green and of the white light are thus in the proportion of about ") 

 1 to 10^'. 



If the isolated eye, which has not shortly before been exposed 

 to strong light, be illuminated by rays of intermediate strength a 

 form of curve is obtained similar to that recorded by previous 

 observers ^). 



The current is led off from the cornea and the posterior surface 

 of the bulbus. The current of rest is compensated 'in the usual way 

 and the connections with the galvanometer are made in such a 



^) Gf. F. HiMSTEDT and W. A. Nac+el. Die Verteiluog der Reizwerte fur die 

 Frosclmetzhaut im Dispersionsspectrum des Gaslichtes, miltels der Aktionsstrüme 

 untersucht. Berichte der Naturforseh. Ges. za Freiburg i. B., XI, 1901, p. 153. 



'■^) The intensities of llie light used will later be communicated in absolute 

 ir.easureinent and at the same time the accurate proportion of the intensities of 

 the green and white will be given. 



3) Gf. for instance Francis Gotch, The Journal of Physiol, 29, p. 388, 1903. 

 Ibid. 31, p. 1, 1904. Hans Piper, Engelmann's Arch. f. Physiol. Suppl. 1905, 

 p. 133. E. Th. von Brücke u. S. Garten, Pfliiger's Arch. f. d. ges. Physiol. 

 120, p. 290, 1907, the latter of whom give a critical review of the literature 

 dealing with the subject. The observers mentioned have all made use of a quickly 

 recording measuring instrument. 



