I believe some of Bouguer’s[194] experiments have been rendered inaccurate—not in their general result, nor in ratio of quantities, but in the quantities themselves—by the difficulty of distinguishing between the two kinds of reflected rays.
[194] Pierre Bouguer, author of, amongst other works, the “Traité d’Optique sur la Gradation de la Lumière.” He was born in 1698, and died in 1758.
[195] The mercury must of course be perfectly clean.
[196] Among other points, I have not explained why water, though it has no shadow, has a dark side. The cause of this is the Newtonian law noticed below, that water weakens the rays passing through its mass, though it reflects none; and also, that it reflects rays from both surfaces.
[197] The review of “Modern Painters” had mentioned the Grotto of Capri, near Naples, as “a very beautiful illustration of the great quantity of light admitted or contained in water,” and on this Mr. J. H. Maw had commented.
[198] The London Review of May 4 contained a critique of the Exhibition of the Society of Water-colors, which included a notice of Mr. Duncan’s “Shiplake, on the Thames” (No. 52). In this picture the artist had painted a rainbow reflected in the water, the truth of which to nature was questioned by some of his critics. Mr. Ruskin’s was not the only letter in support of the picture’s truth.
[199] The present letter is the earliest in date of any in these volumes.
[200] See note to p. 182.
[201] In the “Notes on the Turner Gallery at Marlborough House,” 1856 (p. 23), Mr. Ruskin speaks of the “pale ineffable azure” of the gentian. The present letter was written in reply to one signed “Y. L. Y.” in the Athenæum of February 7, 1857, in which this expression was criticised. In a subsequent issue of the same journal (February 21) Mr. Ruskin’s querist denied the ignorance imputed to him, and still questioned the propriety of calling the gentian “pale,” without at the same time distinguishing the two species.