THE APPLICATION OF QUANTITATIVE STIMULUS 35 



revolution can be given to it by adjustment of the clock- 

 governor, or by changing the size of the drivjng-wheel. The 

 galvanometer spot is thrown down on the drum by the 

 inclined mirror M. The galvanometer deflection takes place 

 at right angles to the motion of the paper ; a stylographic 

 pen attached to a carrier rests on the writing surface. The 

 carrier slides over a rod parallel to the drum. As has been 

 said before, the galvanometer deflection takes place parallel 

 to the axis of the drum, and as long as the plant rests un- 

 stimulated, the pen, remaining coincident with the stationary 

 galvanometer spot on the revolving paper, describes a 

 straight line. If, on stimulation, we trace the resulting 

 excursion of the spot of light, by moving the carrier which 

 holds the pen, the rising portion of the response curve will' 

 be obtained. The galvanometer spot will then return more 

 or less gradually to its original position, and that part of the 

 curve which is traced during this process constitutes the 

 recovery. The ordinate in these curves represents the 

 electro-motive variation, and the abscissa the time. 



We can calibrate the value of the deflection by applying 

 a small known E.M.F., say of *i volt, to the circuit, and 

 noting the deflection which results. This gives us the value 

 of the ordinate. The value of the abscissa which represents 

 time is determined by the distance through which the 

 recording surface moves, in unit time. In this simple 

 manner accurate records are obtained. It has the additional 

 advantage of enabling the observer to see at once whether 

 the specimen is suitable for the purpose of investigation. A 

 large number of records might be taken by this means, in a 

 comparatively short time. 



It is also easy to take the records photographically by 

 wrapping a photographic film round the recording drum. 



I give in fig. 26 a series of responses taken from the 

 root of radish (Raphanus sativus], in which the stimuli were 

 applied at intervals of one minute. This shows how ex- 

 tremely uniform the responses may be rendered, if proper 

 precautions are taken. It may here be once more pointed 



