100 MR. JOHN BURKE ON THE CHANGE OF 



The coefficients of absorption a and & are given by the equations (see p. 89) 



a. = 1 (y + a) ; 

 = 1 - (y + &). 



Hence 



r -f a = 0-545, 



r + 6 = 0-214, 

 and the increase in the absorption, or the difference between a and b, = 0'331. 



INFLUENCE OF TEMPERATURE. 



It does not appear that temperature can have any effect on the phenomenon except 

 either by weakening the intensity of the fluorescence of the transmitting cube A x or 

 A'j by heating effects due to the spark, or by a change of absorption due to tempera- 

 ture directly. But, firstly, since all the fluorescing cubes are uniformly illuminated, 

 and as we are merely concerned with the ratio of the intensity of the light emitted 

 from a l and a\ respectively, the heating being the same for all the illuminated cubes 

 on the one hand and for all the unilluminated ones on the other, the actual intensity 

 E emitted by each cube, which may possibly be diminished by a rise of temperature, 

 does not enter into the results. Secondly, in the eye observations, the various com- 

 binations of screening were brought about with considerable rapidity, sufficient to 

 prevent the possibility of any such effect due to changes of temperature arising. 

 No doubt some of the light from A, is absorbed by A! in the determination of a and /3 

 for Aj ; any change of temperature, however, by this means would necessarily be 

 slow, arid consequently its effect, if any, small. 



Photographic Method. 



The following set of experiments was first carried out. 



It was afterwards found not to be altogether satisfactory on account of the neces- 

 sity in these cases of superposing two photographs, and taking the resultant effect 

 to be proportional to the sum of the separate ones. The experiments, however, are 

 of some interest in revealing this fact : that when considerable differences in inten- 

 sities have to be dealt with the resultant effect of superposing two photographs is 

 not proportional to their sum. (It has not been considered necessary to reproduce 

 these particular photographs here.) Captain ABNEY has previously shown that this 

 is so. 



The first photograph taken, which we shall call Photograph A, consisted of five 

 successive images, on the one plate, corresponding respectively to a gradual alteration 



