388 Einthoven and Jolly 



from tho' battery to the electromagnet is interrupted in two places. The 

 first place lies near the galvanometer, where the circuit can be opened 

 and closed by the observer by means of a key. The second place lies near 

 the rotating discs, where the circuit is closed automatically for a short 

 time after every five revolutions of the pair of discs. 



During the time when the circuit is thus automatically closed, both the 

 discs and the screen are in a position to permit of the rays passing. 



In this way the observer is easily able to illuminate the eye only once 

 during a definite short period. He may close at any moment the circuit 

 of the electromagnet, and then merely wait until the electromagnet comes 

 automatically into action. So soon as this has done its work, which is 

 accompanied by a clicking sound, easily audible, the observer opens the 

 circuit again. 



If it is desired to radiate the light during a longer period, the pair of 

 discs are placed at rest, while the light is allowed to pass freely by the 

 opening in the circumference which is made large. The action of the 

 magnet is then dependent alone upon the closing of the circuit by the 

 observer. 



An electrical contact arrangement is connected with the screen, by means 

 of which a current is made at the exact moment at which the light passes, 

 and is broken at the exact moment of interception of the light. This 

 current sets in motion a signal recording these times photographically. 



The signal resembles a small string galvanometer, and consists of a 

 silver strip 120 mm. long, 143 /x broad, and 6 4/* thick, stretched between 

 the poles of a permanent magnet. It registers the make and break of tiie 

 current with a latency which has been found by previous measurements 

 not to exceed 0001 sec. 



7. The Moist Chamber and the Connection of the Eye with 

 the Galvanometer. 



In a large, well-illuminated room, where the galvanometer is arranged, 

 there is constructed a small dark room. This consists of a frame of wood 

 covered with linen and paper. Its base is rather more than one square 

 metre, and its height rather more than two metres. Within this dark room, 

 resting upon a stone pillar, is the moist chamber containing the eye. The 

 moist chamber, as is usual, consists for the most part of glass, and has an 

 opening in front through which the tube of the microscope is inserted. 



This tube reaches the wall of the dark room, and is here closed by the 

 lens Lg and the diaphragm Dg (fig. 1). Everything in front ^ of Dj, and thus 

 outside the dark room, is covered with pasteboard tubes and black cloth*^ so 

 that no light can enter the dark room through Dg, other than what is sent 

 from the crater through the slit of the collimator. 



• Following Helraholtz, we designate the direction towards the light source, forward, 

 away from the light source, backwartl. 



