BLUENESS, that quality which deno minates a body blue, depending on such a size and texture of the parts that com pose the surface of a body, as dispose them to reflect the blue or azure rays of light, and those only, to the eye.
With respect to the blueness of the sky, M. de la Hire, after Leonardo da Vinci, observes, that any black body, viewed through a thin white one, gives the sensation of blue ; and this he assigns as the reason of the blueness of the sky, the immense depth of which, being whol ly devoid of light, is viewed through the air, illuminated and whitened by the sun. For the same reason, he adds, it is, that soot mixed with white makes a blue ; for white bodies, being always a little trans parent, and mixing themselves with a black behind, give the perception of blue. From the same principle 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 ; for red, he ob serves, unless viewed in a clear, strong light, appears a dark brown, bordering on black : being then in a kind of obscurity in the veins, it must have the effect of a black ; and this, viewed through the membrane of the vein and the white skin, will produce the perception of blueness.
In the same way did many of the early writers account for the phenomenon of a blue sky ; hit, in the explanation of this phenomenon, Sir Isaac Newton observes, that all the vapours, when they begin to condense and coalesce into natural parti cles, become first of such a bigness as to reflect the azure rays, before they can constitute clouds of any other colour. This, therefore, being the first colour which they begin to reflect, must be that of the finest and most transparent skies, in which the vapours are not arrived to a grossness sufficient to reflect other co lours.
M. Bouguer, without having recourse to the vapours diffused through the at mosphere, in order to account for the flection of the blue-making rays, ascrib;ia it to the constitution of the air itself, whereby these fainter coloured rays arc incapable of making their way through any considerable tract of it : and he ac counts for those blue shadows, which were first observed by M. Buffon in the
year 1742, by the aerial colour of the at mosphere, which enlightens these sha dows 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.
The Abbe Mar eas, in a Memoir of the Society in Berlin, for the year 1752, ac counts for the phenomenon of blue sha dows, by the diminution of light ; having observed, that of two shadows which were, cast upon a white wall, from an opaque body illuminated by the moon, and by a candle at the same time, that which was enlightened by the candle was reddish, and that which was enlightened by the. moon was blue. However, the true cause of this appearance seems to be that as signed by M. Bouguer, which agrees with the solution given of it about the same time by Mr. Melville. But, instead of attributing the different colours of the clouds, as Sir Isaac Newton does, to the different size of those globules into which the vapours are condensed, Mr. Melville supposes, that the clouds only reflect and transmit the sun's light ; and that, ac cording to their different altitudes, they may assume all the variety of colours at sun-rising and setting, reflecting the sun's incident light, as they receive it through a shorter or longer tract of air ; and the change produced in the sun's rays by the quantity of air through which they pass, from white to yellow, from yellow to orange, and lastly to red, may be um, derstood agreeably to this hypothesis, by applying to the atmosphere what Sir Isaac Newton says concerning the colour of transparent liquors in general, and that in the infusion •of lignum neph•iticum in particular.