Horizontal Candle-Power of Incandescent Lamps. 215 



intensity corrected for reflexion when the lamp is tilted to 

 various angles is as follows : — 



Angle of 



Inclination . 



o 





 15 



30 

 45 



60 



75 



Corrected Value of Intensity of 

 Light emitted per unit of length. 



2-33 x 26-1 = 1-00 

 2-33x25-9= -98 



The figures in the 

 intensity of the light 



2-33x26-1 

 2*27x26-1 



2-33 x 26-1^-2-33 x 25'8=r00 

 2-15 x26-l-f-2-33x 25-2= -96 

 1-96 x 26-1^2-33 X 25-1= -m 

 1-06 x 26-1^2-33 X 16-7= -71 

 last column therefore represent the 

 sent out by unit length of the incan- 

 descent filament at various angles of the normal. If the 

 cosine law were strictly fulfilled these values should all be 

 unity. 



The easiest way to determine the effect of this departure 

 from the true cosine law on the mean spherical candle-power, 

 is to describe a semicircle (firm line) of unit radius and to 

 set off on its diameter distances from the centre C (see fig. 3) 



Fie-. .3. 



o 



3^ 



**, 



W/ 



ACS 

 proportional to the sines of the angles 0°, 15°, 30°, &c, and 

 then through these points to draw ordinates (dotted lines) to 

 the diameter of the semicircle. Fractions of these ordinates 

 equal to the fractions in the last column of the above table 

 are then set off, and the upper ends of these define a curve 

 (dotted line) which is the Rousseau diagram of the photo- 

 metric curve of luminous radiation of the filament. 



If the value of the mean ordinate of this last (dotted) curve 

 is taken, it gives us the ratio of the mean spherical to the 

 mean horizontal candle-power of the straight filament. For 

 the (dotted) curve as drawn in fig. 3 delineating the obser- 

 vations made with the above-mentioned carbon filament, this 



