Candle-Power of Incandescent and Arc Lamps. 141 



This screen is provided with windows opposite the mirrors 

 fitted with slides, so that each pair of mirrors may, if desired, 

 be used separately : this arrangement is of great service 

 during the setting of the mirrors. 



The position of the sliding pair is read on a scale arranged 

 on the box-shaped screen protecting them from stray light. 

 This scale is graduated to read off directly the distance from 

 lamp to photometer. 



The above system of screens is of course only suited for 

 use in a dark room, but, with a simple arrangement of 

 curtains, no difficulty should be found in using the apparatus 

 in broad daylight. 



An approximate idea of the dimensions of the apparatus 

 may be formed from the scales included in fig. 1 and tig. 3, 



It will be seen that the formula given above for the deter- 

 mination of the mean spherical candle-power depends for its 

 validity upon the truth of the cosine relation, and also on the 

 identity of the reflexion coefficients of the various pairs of 

 mirrors. 



Taking first the cosine relation, this was investigated in 

 the following manner : — 



The mirrors on one side of the upper quadrant of the 

 apparatus having been removed, a lamp-holder was devised 

 to slip on the rods of the mirror supports and to be clamped 

 with a set screw in such a position that a lamp placed in the 

 holder in a radial direction pointed directly head-on to the 

 photometer disk. 



A 32 C.P. lamp was used for this purpose. 



A 16 C.P. lamp, which had been previously calibrated, was 

 then placed in the rotator and, all the window-slides having 

 been closed, the 32 C.P. lamp was placed on each of the 

 mirror supports in turn and successive balances obtained, and 

 the corresponding scale-readings taken. 



From these readings, and a knowledge of the reflexion 

 coefficient of the sliding pair of mirrors, the illumination of 

 the photometer screen was deduced and the curves of fig. 4 

 obtained : the full-line curve taken from the experimental 

 results, and the dotted curve showing the values which would 

 have been obtained had the cosine relation held. 



From these two curves fig. 5 follows immediately, and 

 shows the percentage variation from the cosine relation for 

 the particular Lummer-Brodhun screen under test. 



The reflexion coefficients of the mirrors were obtained by 

 the usual method on an ordinary photometric bench, each 

 pair of mirrors being tested together. 



