[184] Forbes died Dec. 31, 1868; Agassiz in 1873; and De Saussure in 1845.
[185] The letter from Forbes to Mr. Ruskin (dated December 2, 1864) was presumably elicited by the allusions to Forbes in Mr. Ruskin’s letter to the Reader of November 26, 1874 (see ante, pp. 259 and 263). “Advancing years and permanently depressed state of health,” ran the letter, “have taken the edge off the bitterness which the injustice I have experienced caused me during many years. But ... the old fire revives within me when I see any one willing and courageous, like you, to remember an old friend, and to show that you do so.”—The second letter speaks of the writer’s “boyish enthusiasm” for Agassiz, an expression to which Mr. Ruskin appends this note: “The italics are mine. I think this incidental and naïve proof of the way in which Forbes had spoken of Agassiz to his class, of the greatest value and beautiful interest.—J. R.”
[186] In the first edition of “Modern Painters” (vol. i. p. 330) it was stated that “the horizontal lines cast by clouds upon the sea are not shadows, but reflections;” and that “on clear water near the eye there can never be even the appearance of shadow.” This statement being questioned in a letter to the Art Union Journal (November, 1843), and that letter being itself criticised in a review of “Modern Painters” in the Artist and Amateur’s Magazine, p. 262 (December, 1843), there appeared in the last-named periodical two letters upon the subject, of which one was from J. H. Maw, the correspondent of the Art Union, and the other—that reprinted here—a reply from “The Author of ‘Modern Painters.’ ”
[187] The passages in “Modern Painters” referred to in this letter were considerably altered and enlarged in later editions of the work, and the exact words quoted are not to be found in it as finally revised. The reader is, however, referred to vol. i., part ii., § v., chap. i., “Of Water as painted by the Ancients,” in whatever edition of the book he may chance to meet with or possess.
[188] See the Artist and Amateur’s Magazine, p. 313, where the author of the letter, to which this is a reply, adduced in support of his views the following experiment, viz.: to put a tub filled with clear water in the sunlight, and then taking an opaque screen with a hole cut in it, to place the same in such a position as to intercept the light falling upon the tub. Then, he argued, cover the hole over, and the tub will be in shadow; uncover it again, and a patch of light will fall on the water, proving that water is not “insusceptible of light as well as shadow.”
[189] In the review of “Modern Painters” mentioned above.
[190] Of the first edition of the first volume of “Modern Painters.” The size of the book (and consequently the paging) was afterwards altered to suit the engravings contained in the last three volumes.
[191] It may be worth noting that the optical delusion above explained is described at some length by Mr. Herbert Spencer (“The Study of Sociology,” p. 191, London, 1874) as one of the commonest instances of popular ignorance.
[192] Of course, if water be perfectly foul, like that of the Rhine or Arve, it receives a shadow nearly as well as mud. Yet the succeeding observations on its reflective power are applicable to it, even in this state.
[193] It must always be remembered that there are two kinds of reflection,—one from polished bodies, giving back rays of light unaltered; the other from unpolished bodies, giving back rays of light altered. By the one reflection we see the images of other objects on the surface of the reflecting object; by the other we are made aware of that surface itself. The difference between these two kinds of reflection has not been well worked by writers on optics; but the great distinction between them is, that the rough body reflects most rays when the angle at which the rays impinge is largest, and the polished body when the angle is smallest. It is the reflection from polished bodies exclusively which I usually indicate by the term; and that from rough bodies I commonly distinguish as “positive light;” but as I have here used the term in its general sense, the explanation of the distinction becomes necessary. All light and shade on matter is caused by reflection of some kind; and the distinction made throughout this paper between reflected and positive light, and between real and pseudo shadow, is nothing more than the distinction between two kinds of reflection.