charged with vapours, the air no longer retains its morning transparency, and the setting of the sun is attended by a fiery tint which greatly mars the tranquil beauty of the spectacle. It is to those vapours that we are to attribute the inflamed appearance of the sky, because they possess the power of transmitting the tints of the first order, and these are of that fiery cast. Were it not for this circumstance the setting of the sun might justly vie with its rising.
Philosophers had long since settled their opinions as to the colours of the sky. These they explained by assigning to the air the property of reflecting the higher colours of the spectrum (violet, indigo, &c.), and that of transmitting the lower, (red, orange, &c.). The explanation was correct so far as it went, but to make it complete the exact quality of the tints should be determined by indicating the order to which they belong. It was necessary also to ascertain how light is affected by the presence of vapours. The considerations which we have just stated will perhaps supply both these deficiencies.
Thrrd and Fourth Rings. — From No. 29 to 38, and from 39 to 44.
These two rings comprise (if I may use the expression) the richest tints. The tints of the first ring are distinguished by their fiery and metallic appearance; those of the second by their transparency and vividness; those of the third and fourth by their intensity, and by the presence of green, which is wanting in the first and second orders. The first appearance of green is in the third order at No. 32: it appears again in the fourth order at No. 41. These two greens differ but little from one another, and are both beautiful in a very high degree: they have a strong resemblance to the green of the emerald. The tints of the third ring do not differ much from those of the fourth: their most marked difference consists in the diminution of transparency observable in passing from the third to the fourth order.
The colours contained in these two series abound in the three kingdoms of nature; the vegetable kingdom however seems to present them in the greatest proportion.
The predominant colours in these two parts of the scale are the red, the green, and the yellow-green. There is here, properly speaking, no species of blue, but its absence is counterbalanced by the presence of the green, which is not to be found in the first two rings. It would seem as if the blue belonged peculiarly to the spacious vault of heaven, and the green to the surface of the earth. They are two dominant colours in nature, but their domains are separated, and the separation seems to me not to be accidental. It was necessary, I suppose, that the atmosphere should be composed of the most subtile particles, in order that they might remain suspended in space; the earth did not require to be of so delicate a texture. Hence we have two very distinct orders
