380 Einthoven and Jolly 



it is difficult to obtaiu consistent results, variations of over 5 per cent, 

 being by no means unfrequent." Petavel made use of a lamp regulated 

 by hand with carbons placed at right angles with one another. To what 

 extent the inconstancy of his results is to be attributed to this circumstance 

 we do not venture to judge. 



We have ourselves made use of a differential lamp of Siemens and 

 Schuckert of 20 amperes, which burns quietly when fed either by a 

 battery of accumulators at 65 volts or by a dynamo at 110 volts with 

 appropriate resistances. At the same time the distance between positive 

 and negative carbon is great enough to permit of our utilising the radiation 

 from the centre of the crater depression. The lamp has occasionaHy 

 during the course of our work shown some irregularity, but this does not 

 often happen, and when it occurs the observation is repeated. The 



D, 



p 



L. 



-f 



Fig. 1. — Spectroscopic arrangement and system of lenses designed to transfer a maximum of 

 spectroscopically isolated rays from the crater of the arc lamp to the pupil of the frog's eye. 



photographic records made of the movements of the string of the galvano- 

 meter, where in the same way only the centre of the crater depression 

 was used, give us no reason to assume that variations of 5 per cent, in the 

 intrinsic brilliancy of the crater light occur with a steady lamp. 



3. System of Lenses. 



As we chiefly desired to make use of light between definite wave-lengths, 

 we employed a spectroscopic arrangement. In constructing this \ve had to 

 solve the question as to how a maximum of spectroscopically isolated rays 

 might be obtained from the crater and transferred to the pupil of the eye 

 to be investigated. 



The solution was found to be as follows: — From the slit S^ (fig. 1) a 

 spectrum is formed by the aid of lens L^ and prism P in S.,. From this 

 spectrum there is cut off" by the octangular diaphragm D.^ a part lying 

 between definite wave-lengths. 



