SPECIFIC GRAVITY OF LIGHT. 145 



wnity, we shall find them expressed as follows at a mean 

 temperature. 



SPECIFIC GRAVITIES. 



Water 1-00000000000 



Air 0*00120000000 



Light o-oooooooooi3 



If this be the real density of light, it will appear, that all Hence incapa- 

 former attempts to appreciate its specific gravity by meeha- J* 1 ^. * vjjf 

 meal means must have been fruitless, as the quantity thrown rectly. 

 by a lens, however large, upon a balance of the most de.i- 

 cate-construction, must be exceedingly minute; yet it may 

 have very considerable effects when exerted upon the body 

 of the planets. May not the diurnal motion of the planets 

 be the effect of its momentum? 



It appears to me, that the experiment on inflected light, Inflection^ 

 mentioned in Newton's optics, performed by passing the ^f a i ne( j b/un- 

 light through an aperture of a window shutter into a dark- duiation than 

 ened room, is much better explained, by allowing an undu- 

 latory than a radiating motion of light. 



It is the nature of all fluids to undulate in circular arcs 

 when moved by any impulse. 



Let a represent an aperture into a darkened room, equal Phenomena of 



• ^ c • i • j* 2. j * flight admitted 



to T V part ot an inch in diameter, 6, e, a, e, j, waves ot t |j rou(rh a 



light, moving in succession against the solid object g h, small aperture. 



which we will suppose the side of a house : here the light, 



meeting with an opake substance, will be reflected every 



where, except at the aperture «, which will then become 



the centre of motion. The undulating light, having passed 



the aperture, will dilate in the concentric arcs 1, 2, 3, 4, &c, 



till they arrive at i, on the opposite side of the room; and 



the greater the distance between a and i, the greater will be 



the diameter of the shadow of the aperture; all obstacles 



placed in this lucid stream will have their shadows augmented 



in diameter, when received upon the wall, in proportion to 



their distance therefrom. 



If the attraction of the sides of the aperture and window Not owing to 

 shutter was the cause of this enlargement of the shadow, j*f ."J^J t- the 

 the obstacle^ when interposed in the lucid stream within the sides of the 

 room, would also attract the light, and the diameter of its a l )erlure « 



Vol. XIX— Feb. 1808. L shadow, 



