1901.] Effect of Light and Electric Radiation on Matter. 165 



But when the radiator was placed at a distance of 15 cm. (see (c) ), the 

 fluctuations almost disappeared, and the galvanometer deflection was 

 held rigid as long as the radiation was kept on ; in fact, we have here 

 an effect which physiologists describe as "tetanic." On the cessation 

 of radiation there was immediate recovery. It will be noticed how 

 •extraordinarily consistent are the succeeding values of response. The 

 resultant effect being due to the additive effects of numerous flashes, an 

 occasional failure of an individual flash has little or no importance. 

 The series of responses in (d) was taken after half an hour, and it will 

 be noticed how very consistent they are among themselves, and how 

 similar to those in (c), showing that even after half an hour's con- 

 tinuous work there had been no fatigue, with the attendant change of 

 ■sensibility. 



Relation between the Intensity of Radiation and the Conductivity-variation. 

 — The resistance of the receiver being not very large, the external 

 resistance of the shunted galvanometer and of the cell are not 

 negligible in comparison, and the variation of deflections is, therefore, 

 not proportional to the variation of resistance. To interpret the 

 absolute values of the deflections, a resistance box was substituted for 

 the receiver, keeping the rest of the circuit just as before. In this 

 way the absolute values of the resistances corresponding to particular 

 deflections were found. Some of these are given on the left-hand side 

 of fig. 5. 



The galvanometer deflections, when the radiator was at distances of 

 40, 25, and 15 cm., were 23, 33, and 42 divisions respectively. Owing to 

 the comparative steadiness of the last two deflections there is no 

 uncertainity about them ; but on account of the fluctuation in the 

 •deflection when the radiator is at a distance of 40 cm., it is difficult to 

 find the exact value of the deflection ; the mean of the various deflec- 

 tions gives twenty-three divisions. The absolute values of resistances 

 corresponding to these deflections are 180, 380, and 1020 ohms. The 

 original resistance being 20 ohms, the variations due to the different 

 radiation intensities are 160, 360, and 1000 ohms. 



The intensities of radiation at the above distances may approxi- 

 mately at least be taken as proportional to » ^ ' > or as 



14 : 36 : 100. The corresponding molecular effects as measured by 

 the increase of resistance are found to be as 16 : 36 : 100. 



It will thus be seen how accurately the indications of the Ag' receiver 

 measure the intensity of radiation. Further progress in the study of 

 different phenomena connected with electric radiation has been seriously 

 hampered owing to the want of means for measurement of intensity of 

 electric radiation. But this difficulty, as will be seen from the above, 

 is not insuperable. 



The strict proportionality of molecular effect can only be taken as 



