30 ON ILLUMINATION. 



nncf in order to determine with prccjslon the relative quantities 

 ol'liglit emitted by the two shades, we must remove the lamp 

 ■whicli bears the shade that emits the most hght until the equa- 

 lity of the shadows be re-established, and then, measuring the 

 <!istances of the lamps from the photometer, the quantities of 

 light will be as the squares of these distances. 



if we wish to know how much light is absorbed and lost in 

 employing any given shade, Ave must operate in the folio \vii:g 

 manner: — Having placed the two lamps, without their shades, 

 at equal distances in front of the photorr.eter, and having equa- 

 lised the flames of the lamps in the manner already described, 

 we place the shade that is to be tried upon o»ne of the lamps, 

 and the equality of the shado\^s is instantly destroyed. In 

 order to re-establish this equality, we remove the lamp that is 

 without a shade, and when it re-established we measure the 

 distances of the tv/o lamj^s from the })hotomcter. The quantity 

 of light emitted by one of these lamps without a ^rade is to 

 that emitted by the same lamp with the shade, as the square of 

 the distance of the lamp that burns without a shade is to the 

 square of the distance of that maslced by the shade. 

 Thepwvrc-r of. The object in view in using Tt shade being to disperse the 



a shade- 1«)> ravs of a too dazzling flame, without destroying them, it js 



soften swi dss.- •; o ' j a ' 



jjerse S'ght, riuI evident th-^t the less the flame of a lamp is apparent through the 



sias of inter- g- j^_^ ^f ^ shade, thequantitv of light emitted being the same, the 



are.4i&rsi«t • belter it wiil answer its purpose, but as we always see, more 



qaaiiiii's. or less distinctly, the brilliant flame of an argand's lamp through 



the shade which masks it, it is evident that a considerable part 



r)( the light emitted by a lamp thus masked does not proceed 



front the shade, but, passing directly through the sides of the 



ab-ade. it comes from the tlume in straigiit lines. 



Jt is Vi'j.ht, coming from the flame to the eyes in straight 

 Iines> which a shade is destined to disperse and to soften ; and 

 as it is C'jrtaJ'.i that tAvo shades of diflerent materials may have 

 an equal ]>owcr in softening the direct rays of the flame of a 

 lunp, and that nevertheless the total quantities of light that 

 they emit may be very diiterent, it is necessary to pay attention 

 to thi;i remarkable circumstance in the choice of shades. 



I'he shades to be coin])urcd should therefore be examined, 



' iirst in regard to their ])owcr of masking and softening the 



tlLrect ia\s of the fl imc of a lamp, an:l alte. wards in regi^rd to 



tl.c 



