ON ILL-UMINATION. g^ 



sequently less dark than that enlightened by the weaker 

 lamp. 



If, instead of csLiblishing an equality in the quantity of light 

 emitted by the two lamps, we would ascertain the relative 

 quantities of light that they emit when their flames are unequal, 

 they must be placed on two tables before the photometer, and 

 after having brought the shadows into contact with each other, 

 we must remove the stronger lamp until the intensity of its 

 light at the vertical field of the photometer be diminished by 

 the increase of its distance, till a perfect equality is established 

 between the densities of the two shadows; and then Ave mea- 

 sure exactly the distance of each lamp from the photometer. — 

 'J'he squares of the distances will be as the quantities of light 

 emitted by the lamps. 



In order to exclude the light reflected from the sides of the 

 room and other surrounding bodies, with a view to render the 

 shadows more distinct, and their comparison more easy, the 

 photometer may be placed in a quadrangular box, open in the 

 front, forming a kind of centry-box, 1 5 or iS inches in height 

 and 10 or 12 inches in width and depth, constructed of boards, i 



oreven of pasteboard, and line:l within and without with black 

 paper. 



The experiments are simple and easy that may be made Valuable re- 

 with this little apparatus for determining the different substances ^""s "lay be 

 tnat may be employed in constructing shades to soften the ments upon 

 light of lamps ; and as this enquiry must lead to very important ^"'^^'^s. 

 improvements, both with respect to economy and to elegance 

 and comfort, I recommend these researches to all those who 

 are occupied in the improvement of lamps. These experiments 

 may be made in the following way: — 



Having prepared two shades, of the same form and dimen- Instructions for 



sions, that are to be comi)ared together, we must beoin by ™^'^"^g ^^^^'^ 

 . . , ,. . ^ r 1 1 experuner.ts. 



placmg the two lamps at the same distance m front of the pho- 

 tometer, and causing them to burn v/ith the same degree of 

 brightness, and then masking the flames of the lamps by the 

 two shades, we must examine anew the shadows. If these 

 shadows are of equal densities, we may conclude that the two 

 shades emit equal quantities of light; if the densities of the 

 shadows arediflercnt, then the shade that enlightens the shadow 

 which is the least dense is that which emits the most light ; 



and 



