ILLUMINATION AND 



439 



The Sharp-Millar Photometer is typical of the portable type. 

 A plan of the instrument is shown in Fig. WM. T is a tube which 

 can swing through 180. At the elbow R is either a mirror or a 

 white diffusing surface. The light entering the tube is reflected 

 at right angles by R and is directed towards P, a Lummer- 

 Brodhun Screen. This light is balanced against the brightness 

 of a screen W, illuminated by a 6-volt tungsten lamp S, which is 

 standardized. The viewing aperture is at O. The screens, Si, 

 S z , S 3 prevent stray light from the lamp N falling on the window 

 W. The balance is obtained by moving the lamp S, which varies 

 the illumination on the window W. By reading the position of 

 the lamp on a scale when balance is obtained, the candle- 

 power can be determined. If illumination is being measured, 

 a white translucent glass, called a test plate, is placed at T and 

 the mirror used at R. The brightness of T 7 is a measure of the 

 total illumination falling upon it. 



On the other hand, if the candlepower of a lamp is being 

 measured, extraneous light must be excluded. Therefore, the 

 test plate at T is removed and 

 the mirror at R is reversed, 

 which substitutes for R tin- 

 white diffusing surface. The 

 only light which enters the 

 tube, is that coming from the 

 source towards which the tube 

 is directed, the extraneou- 

 light being cut off by the -ide> 



of the tube. 



If the light to be measure* 1 



,- '"" l " 1 - 1 " f " r I'"''K " c ,;. f ,, r 



balance, either screen A or A i standard lamp of Sharp-Millar 

 maybe interposed between /.' " I0tor - 



and the photometer head, thus rediicinu the light in a k 

 ratio. On the other hand, if the measured liirht i- found to be 

 too dim, these scree n> may be turned so as to lie between H 

 and the photometer head, thus reducing the light from tin- 

 standard, and making a balance possible. 



The M.mdard lamp N i> supplied by a (i-volt storage bat 1 

 Its candlepower is controlled by a rfaottat The correct adjust- 



