Candle-Power of Incandescent and Are Lamps. 1VS 

 or, putting in the values of the constants : 



MSCP= — x (— Y x — vMHCP 



l ' (58-5 x V d ) X 2(11 + 1) x^ 1 - 11 -^. 



5*09 

 = ~ xM.H.C.P. 



d- 



M.S.C.P. 5-09 . . . . ■ , 

 M.H.C.P.^ ^ where <* ism feet. 



Hence a scale could be easily constructed from which the 



ratio t rrT * \ ' might be read off directly. 

 M.H.C.P. & 



Before passing on to the results of the tests, some interesting 

 figures relating to the absorption of ground glass might be 

 quoted. 



In designing the apparatus described above, attempts were 

 made, at the suggestion of Dr. Fleming, to diminish the 

 dimensions of the integrating photometer by fixing ground- 

 glass screens in the windows between the mirrors in order to 

 absorb a portion of the light and so reduce the illumination 

 on the photometer-screen, which with an instrument of small 

 radius would otherwise be uncomfortably brilliant, especially 

 when dealing with arc lamps. 



The validity of this device must of course depend upon the 

 constancy of the absorption coefficient of the screen when 

 illuminated by lights of very different spectral composition, 

 and the following experiments were made to investigate this 

 point. 



The screen used was a sheet of glass of an average thick- 

 ness of *08 inch, ground by sand-blast on both sides. 



This screen was set up in a vertical position, 25 inches 

 from the photometer-screen and at right angles to the 

 photometric bench. The source of light under test was set 

 up behind the screen at a fixed distance from it, a photometric 

 balance being obtained by means of a lamp, on the other 

 side of the photometer, whose distance could be adjusted. 



The sources used were a 50 C.P- incandescent Focus lamp, 

 a 100 watt Xernst lamp, and a hand -fed continuous- 

 current arc. 



However, the results obtained with the Arc lamp can 

 scarcely be compared with the others, as the distance of the 

 lamp from the screen was much greater in this case than in 

 the others, due to the intensity of the light. In the case of 

 the Focus and Xernst lamps the distance from lamp to screen 

 was '1') inches. 



