LATENT PERIOD AND REFRACTORY PERIOD 267 



balanced key, K 1} completes the circuit, and actuates the 

 shutter. 



A thread connects the shutter with one arm of a second 

 balanced key, K 3 . This arm is so overweighted that when 

 freed, it causes two prongs, at the opposite extremity of the 

 lever, which complete the primary circuit of the Ruhmkorff's 

 coil, to be lifted out of their cups of mercury, and thus the 

 circuit is interrupted. But the thread is of such a length 

 that when the shutter is set, so as to close the pinhole, 

 the prongs, dipping into the cups of mercury, complete 

 the primary circuit. The overweighted arm of K 3 falls, 

 with the drop of the shutter lifting up the prongs, and 

 thus suddenly interrupts the primary current, giving rise 

 to a break-shock in the secondary, which passes through 

 the plant. During the course of the preliminary adjustment, 

 when primary circuit of the coil is made, a make-shock is 

 produced, but this is prevented from affecting the plant by 

 a key, K 4 , which short-circuits the secondary. When the 

 adjustment has been made, this short-circuiting key is opened. 



Briefly to recapitulate the procedure : The drum, carrying 

 the sensitive film, is released, and begins to revolve. The 

 key K 2 of the shutter-circuit is kept open, until a uniform 

 speed is attained. It is then closed. The striker connected 

 with the drum now closes the balanced key, K, ; the shutter 

 drops, and simultaneously interrupts at K 3 the primary current 

 of the induction coil, thus causing an excitatory shock to be 

 given to the plant. 



The determination of the latent period. — It will be seen 

 from the upper of the two photographs, given in fig. 1 10, 

 that for a period of y 2 -^ of a second the record remains hori- 

 zontal. This represents the latent period, after which the 

 tissue begins to respond. For a further period of half a 

 second the leaf is seen to fall with a considerable and approxi- 

 mately uniform speed. The rate of movement of the tip of 

 the leaf is now 71 mm. per second. After this the leaf con- 

 tinues to fall, but with a diminishing speed, till the maxi- 

 mum contraction fall is reached. From records obtained on 



