192 Scientific Proceedings, Royal Dublin Society. 



light. Blackened tubes are necessary to prevent tlie intrusion of extraneous 

 light into the photometer, which sliould be of considerable lengtli to diminisli 

 the error arising from the magnitude of the trausliicent screen. But tlie 

 results so obtained are not very satisfactory, owing to the unavoidable reduction 

 aud consequent feebleness of the source of light thus measured. 



It may therefore be of interest to bring before the Society two other 

 methods of comparing the scattered liglit with some standard source of light, 

 and reducing the illuminating power of tlie stronger in some other way than 

 by varying the distance. 



Method A consists of a rapidly revolving opaque disc with a transparent 

 sector that can be altered in size, and its angular magnitude measured. The 

 apparatus shown in fig. 1 can be driven by hand ; a simple speed-gear is all 

 that is necessary. It is placed at a given distance between the reflecting 

 surface, which is illuminated by the sun or strong artificial light, aud the 

 photometer. The widtli of the sector is altered until equality of illumination 

 between the reflecting surface and a standard source of light is obtained as 

 shown by some transmission photometer, such as Bunsen's, Joly's, or Lummer 

 and Brodhun's. If the scattered light is coloured, as from a brick building, 

 in front of the standard light, a wedge of suitably coloured glass, or a 

 coloured gelatine film of increasing thickness, is gradually interposed until 

 a similar tint is obtained. This method of diminishing the iutensity of 

 light by a revolving sector of variable size is well known, and has been 

 used for other purposes by Swan in 1849, and Abney in 1890 ; but, so 

 far as I know, it has not hitlierto been employed for the purpose I have 

 described. 



Tiiere is another application of the revolving sector which is very useful 

 for teaching-purposes, and possibly has been so used, but I have not met 

 with it. Two equal sources of light are balanced against each other by 

 means of, say, a Joly photometer. The revolving sector is placed before one, 

 and the opening iuljusted in successive experiments to, say, 180°, 120°, 90^ 

 and 60°, so that on rapid revolution one-half, one-third, one-fourth, and oue^ 

 sixth of the light on that side falls on the photometer. The distance the 

 other light, or the photometer, has to be moved to restore equality of 

 illumination in each case, demonstrates with ease and accuracy the law of 

 inverse squares. Thus, if the sector be set to 90°, one-fourth of the light 

 is transmitted by the I'evolviug disc ; and the photometer has to be 

 placed twice as far from one caudle as the other in order to obtain equality 

 of illumination. This experiment is so useful and interesting for young- 

 students that I have added it to the regular course of introductory practical 

 physics in my laboratory. 



