OPTICAL INSTRUMENTS. 



109 



For measuring the intensity of light various photometers have 

 been invented, such as that of Rumford, in which the illuminations 

 of two shadows of the same object produced by two sources of 

 light are compared; that of Bunsen, in which the light trans- 

 mitted through stearined paper is compared with that from 

 another source reflected from unoiled paper. In Bouguer's 

 photometer comparison is made of the brightness of two portions 

 of a surface, separately illuminated by light from two sources j one 

 of the sources is then removed further off or nearer until the 

 illumination is equal. Ritchie's photometer is based on the 



same principle, and somewhat simpler. To these others might be 

 added, but the majority are modifications of the above. Wheat- 

 stone suggested a form depending upon the relative colours of 

 polarised light ; but as the eye is more sensitive to differences of 

 intensity than to those of tint the principle does not seem promis- 

 ing. Recent investigations on the action of light upon the 

 electric conductivity of selenium, seem to show that this action: 

 may conversely be used for photometry. Lastly, Mr. Crookes 

 has proposed to use his radiometer, mentioned below, for the same 

 purpose. 



