90 Mr. G. J. Burch. On determining tlie, Value of [May 22, 



employment of photography for the production by means of the 

 capillary electrometer of records of the electrical phenomena ac- 

 companying the excitation of the leaf of Dioncca. Other physiologists 

 have availed themselves of this method for the purpose of determining 

 the times at which the electrical changes in living tissues begin and 

 end, but no complete investigation has been made of the relation 

 between the curves obtained and the amount of variation of the 

 difference of potential of which they are the expression. 



A series of experiments which I undertook two years ago, at the 

 suggestion of Professor Burdon Sanderson, for the purpose just in- 

 dicated, has led to the discovery of a method by which it is possible 

 to determine from the photographed curve the difference of potential 

 corresponding to any part of its course. 



I propose in this preliminary ifote to state briefly the experimental 

 results upon which the method is based, to describe the application 

 of it in its simplest form to the analysis of a photographed curve, and, 

 finally, to indicate the corrections which have to be applied to the 

 approximation thus obtained. 



I. The Normal Curve. 



The magnified image of the capillary, pointing vertically down- 

 wards, is projected through a slit upon a sensitive plate travelling 

 horizontally at a uniform rate. During the passage of the plate 

 behind the slit a sudden permanent difference of potential of known 

 value is introduced between the terminals of the electrometer, causing 

 the meniscus to move from its original position of rest to another 

 level, the movement being recorded on the plate by the curved 

 outline of the shaded portion. This will be referred to as the 

 "normal curve," and the movement as a " normal excursion." 



(1.) By direct photographic evidence I have proved that there is 

 no measurable delay between the communication of a difference of 

 potential and the commencement of the movement of the meniscus. 



(2.) Under ordinary circumstances the meniscus ceases to move the 

 instant the source of electromotive force is withdrawn. [See 

 also (7)]. 



(3.) The normal curve is of the same form and dimensions so far 

 as it extends, no matter what may have been the difference of poten- 

 tial which produced it. The accompanying diagram illustrates a 

 normal curve taken with a large resistance, and low down in the 

 capillary, so that variations of resistance during the excursion 

 [see (7)] may be neglected. The entire curve corresponds to a 

 difference of potential of O'Ol volt. The portion above the line 

 through 5 is the entire normal curve for 0'005 volt, and the portion 

 above P is the curve for O00195 volt. 



