140 Mr. Dyke on the Determination of the Mean Spherical 



Now if D = total distance in feet by any one path from 



source to screen, 

 and K = reflexion coefficient of a pair of mirrors, 

 and if the cosine relation holds, 

 then the intensity of illumination of the screen in candle 



feet =^ 2(I fl cos 6). 



T7" _ 



Hence M.S.C.P.= % LW W t , X illumination of screen 



D " *(n + l) inO.F. 



The following description briefly outlines the construction 



mo np 



of the apparatus designed to obtain the ratio MU ' * * . 



lVl.xi.O.Jr . 



A wrought-iron band, 4 in. X J in., A (fig. 3), stiffened with 

 a small L section, is bent into a semicircle 6 feet in diameter, 

 and fixed by wood supports with its diameter vertical. 



Eleven bosses, B, fitted to the outside of the semicircle, 

 carry the mirror supports C ; these supports slide radially 

 through the bosses and may be fixed in any position by set 

 screws. 



Each mirror support carries a pair of brass plates to 

 which mirrors are clipped, each plate being independently 

 adjustable. 



The arrangement is shown in PL III. fig. 3. The distance 

 from centre to centre of the mirrors of each pair is 8 j inches. 



The lamp to be tested is mounted on the rotator at D, at 

 the centre of the semicircle and in the plane of one set of 

 mirrors. 



The photometer, of the Lummer-Brodhun pattern, is placed 

 at E, also at the centre, but in the plane of the other set of 

 mirrors. 



Both lamp and photometer are so mounted as to allow of 

 their being set accurately to the central position. 



The photometer has a 1-inch disk, and the case is cut away 

 at F, so as to permit light to reach the whole surface of the 

 disk from any angle in the vertical plane containing the 

 disk. 



The object being to find the relation between the mean 

 spherical and mean horizontal candle-powers, the side G of 

 the photometer must be illuminated with an intensity pro- 

 portional to the mean horizontal candle-power. 



This is done by arranging the pair of mirrors H to slide 

 in a horizontal plane along a railway J formed of an inverted 

 T section. 



A matchwood screen K prevents any light reaching the 

 photometer except by way of the mirrors. 



