Horizontal Candle-Power of Incandescent Lamps. 



209 



intensity of the light emitted or the candle-power of the 

 filament in a horizontal direction F H, and let I be the in- 

 tensity in any direction FN making an angle NFH = # with 

 the horizontal direction. Let I s be the mean spherical 

 candle-power. 



Fit*. 1. 



Let us then start with the assumption that I = Ir- cos 6. The 

 justification for this is found in the fact that if a long straight 

 incandescent filament is placed behind a small rectaugular- 

 shaped hole in a metallic plate, then inclining the filament to 

 the plane of the plate within certain angular limits makes no 

 sensible difference in the quantity of light which comes 

 through the hole *. Consider then a zone of the sphere 

 swept out by an elementary arc NN' of the semicircle. The 

 surface of this zone is 27rr 2 cos 0dO, where dO denotes the 

 angle N'FN. 



Hence the quantity of light falling on the whole zone is 

 Iii27rr 2 cos 2 Od0, and the total quantity of light incident on the 



* For the limits within which the above statement is confirmed by 

 experiment the reader is referred to the Appendix to this Paper. 



Phil. Mag. S. 6. Yol. 10. No. 56. Aug. 1905. P 



