382 



EECEXT PROGRESS IN PHYSICS. 



Fig. 81. 



We now pass to the particular object of Masson's investigation. 

 The arrangement of his experiments was essentially as follows : A 



rotating disk, a b, fig. 81, (the rota- 

 tion being produced by clock-work,) 

 divided into white and black sectors, 

 as in fig. 79, was illuminated in the 

 direction of ^ (7 by the constant light 

 of a lamp L, which was movable in the 

 line of this direction. This lamp was 

 placed in a black case so that it could 

 throw its light on the rotating disk 

 only through a tube. In the direc- 

 tion of the line B C a, movable spark 

 micrometre F was placed. One of the 

 knobs of this micrometer was in con- 

 ducting connexion wi:h the upper 

 coating of a horizontal glass plate, the 

 other knob with the lower coating; 

 the spark always passed between the 

 two knobs as soon as the charge of the plate had reached a certain 

 limit, which depended upon the distance of the knobs from each other. 

 Masson first satisfied himself that, for the instantaneous light of the 

 electrical spark, the intensity of the illumination was also, as in other 

 cases, in the inverse ratio of the square of the distance. 



The lamp L being at a given distance from the disk a h, the spark 

 micrometer was gradually removed from the disk, until at the passage 

 of the spark the sectors of the rotating disk were no longer distin- 

 guishable, and the distance of the spark from the disk was determined. 

 The lamp was then moved, and the same experiment repeated, the 

 distance between the knobs of the spark micrometer remaining un- 

 changed. The following table gives the results of such an experi- 

 mental series ; Z denotes the distance of the lamp, Y the correspond- 

 ing distance of the spark micrometer from the middle of the disk ab: 



Since Z and Y increase in an equal (or very nearly equal) ratio, it 

 is evident that, with increasing distances, the illumination for both 

 sources of light decreases according to the same law; hence the illu- 

 mination by an electrical spark is likewise inversely proportional to 

 tiie square of the distance. 



