BLUENESS

, that quality of a body, as to colour, from whence it is called blue; depending on such a size and texture of the parts that compose the surface of a body, as disposes them to reflect only the blue or azure rays of light to the eye.

The blueness of the sky is thus accounted for by De la Hire, after Da Vinci; viz, that a black body viewed through a thin white one, gives the sensation of blue, like the immense expanse viewed through the air illuminated and whitened by the sun. For the same reason he says it is, that soot mixed with white, makes a blue; for that white bodies, being always a little transparent, when mixing with a black behind, give the perception of blue. From the same principle too he accounts for the blueness of the veins on the surface of the skin, though the blood they are filled with be a deep red.

In the same manner was the blueness of the sky accounted for by many other of the earlier writers, as Fromondus, Funceius, Otto Guericke, and many others, together with several of the more modern writers, as Wolfius, Muschenbroek, &c. But in the explication of this phenomenon, Newton observes that all the vapours, when they begin to condense and coalesce into natural particles, become first of such a magnitude as to reflect the azure rays, before they can constitute clouds of any other colour. This being therefore the first colour they begin to reflect, must be that of the finest and most transparent skies, in which the vapours are not yet arrived at a grossness sufficient to reflect other colours.

Bouguer however ascribes this blueness of the sky to the constitution of the air itself, being of such a nature that these fainter-coloured rays are incapable of making their way through any considerable tract of it. And as to the blue shadows which were first observed by Buffon in the year 1742, he accounts for them by the aerial colour of the atmosphere, which enlightens these shadows, and in which the blue rays prevail; whilst the red rays are not reflected so soon, but pass on to the remoter regions of the atmosphere. And the Abbé Mazeas accounts for the phenomenon of blue shadows by the diminution of light; observing that, of two shadows which were cast upon <*> white wall from an opaque body, illuminated by the moon and by a candle at the same time, that from the candle was reddish, while the other from the moon was blue. See Newton's Optics pa. 228, Bouguer Traité d'Optique pa. 368, Edinb. Ess. vol. 2 pa. 75, or Priestley's Hist. of Vision &c, pa. 436.

BOB of a Pendulum, the same as the ball, which see.