si 



ON ILLUMINATION. 



wiilch it emits may be sufficiently strong to hurt the eyes, 

 especially when viewed at a small distance. 



. The size and intensity of the flame being the same, the in- 

 tensity of the light emitted by the surface of a shade that masks 

 it will be as that surface, and consequently as the square of the 

 diameter of the shade inversely. 

 The larger the 1l the intensity of the light emitted by a shade four inches 

 L'minousiuTiU '" <^'iametev be equal to four, it will be reduced to one on 

 api)ear when doubling the diameter of the shade, and that without any 

 - ^ •- '■ •• change in the total quantity of light which is diflhsed in the 

 "■' room. This shews the advantage to the eyes that will result 

 from the use of shades of large dimensions, 

 Smalt sha-les The small spherical shades of roughened glass which are 

 appear Jaz^l'.ng. sometimes employed for lamps, have been found to emit a 

 light too dazzling to the eyes. In order to remedy this in- 

 CQnvenieiicc, all that is necessary is to make the shade larger. 

 If these globes are more dazzling than globes of crape or 

 gauze of the same dimensions, that circumstance proves no 

 more than that roughened glass absorbs less light than these 

 silk stufis do; and fioni this we may conclude that the solid 

 parts of silk are less transparent than those of glass, and con- 

 sequently that this substance is less fit to be used in making 

 shadcc- for lamps than glass. 

 Greataclvan- I will just mention here a circumstance respecting roughened 



ta^e ot glazing glass, which although not immediately connected with the 

 windows with *= . . '^ -^ „ . , . 



roughened glass subject of this paper, appears nevcitheless sutnciently import- 

 when the ilhi- .^.^^ j:q (Jesn,-ve attention. It often happens, in great towns, 

 mmationisvery , ,. , , , • • , 



obliquely re- that a room has no other hglit than what it receives by win- 



«eived, as la dows which look into a small court, surrounded on all 

 rourts, alleys, .,,,.,,.,,. . , , ,, , 



jtc. sides by high buildings : in tnese cases the room would be 



more copiously and better lighted by panes of roughened 

 glass than by transparent panes. I'he rays of day-light, 

 which descend almost perpendicularly from above into the 

 cd^irt, fall upon the panes at so small an angle of incidence 

 that, when the exterior surface of the glass is polished, they arc 

 r- ■: in great part thrown ofF by reflection, and do not get into the 



room ; and even those which, not being reflected, pass through 

 the pane, as they fall directly upon the floor, where they are 

 almost all absorbed, the objects ia the room are very little en- 

 lightened j but when the pan; is roughened, the asperities of 



the 



