406 



Einthoven and Jolly 



pure line -which appears during the lighting period I, and in which every 

 trace of rhj'^thmical oscillation fails.^ It is to be noted that the illumination 

 employed here, where the slit of the collimator is used, is derived from the 

 central part of the crater, but we cannot altogether exclude the possibility 

 that the arc lamp has burnt irregularly during the recording of fig. 22. In 

 that case the oscillations of potential difference in the eye during illumina- 

 tion would have their natural origin in the rhythmically varying intensity 

 of the light stimulation. But we did not observe an irregular condition 

 of the lamp during the experiment, and must not, therefore, overlook the 

 other possibility that it has been burning quietly. The oscillations in 

 potential difference of fig. 22 would then have their origin in the rhythmical 

 reaction of the eye itself. 



Their failure in fig. 15 may, if the latter explanation is adopted, be 



Fig. 22. — Rhythmical reaction to a continuous stimulus. Li^rht eye. Absc. 1 mm. =0"2 sec. 

 Ordin. 1 mm. = 15 microvolts. White light. Intensity of illumination = 02 Iw. I, light; 

 d, darkness. 



attributed to the rest of a quarter of an hour in darkness which has been 

 given to the eye, during which it may be supposed to have recovered. 



5. The Latent Period. 



As we have mentioned when describing our method of investigation, 

 the screen which intercepts the light rays is automatically moved by a 

 strong spring set in action by means of an electromagnet. The movement 

 of the hand required in closing the circuit of the electromagnet and the 

 movement of the armature of the magnet itself give rise to slight jerks 

 whereby the signal line and the string itself are sometimes set slightly in 

 movement. One or two tenths of a second later, at the exact moment at 

 which the light enters the eye or at which it is intercepted, a circuit is 

 made or broken, by means of which .the signalling instrument receives its 

 current. This instrument itself has, as we have "already mentioned, a 



* If we study the curve with a magnifying glass we observe very fine rhythmical 

 oscillations of about 25 or 30 periods per second. These are caused by the technical 

 deficiency of the recording apparatus, and have no bearing upon changes in potential 

 diflference which might occur in the eye. 



