The Storm-Cloud of the Nineteenth Century. Ruskin John

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15

Diffraction.—Since these passages were written, I have been led, in conversation with a scientific friend, to doubt my statement that the colored portions of the lighted clouds were brighter than the white ones. He was convinced that the resolution of the rays would diminish their power, and in thinking over the matter, I am disposed to agree with him, although my impression at the time has been always that the diffracted colors rose out of the white, as a rainbow does out of the gray. But whatever the facts may be, in this respect the statement in the text of the impossibility of representing diffracted color in painting is equally true. It may be that the resolved hues are darker than the white, as colored panes in a window are darker than the colorless glass, but all are alike in a key which no artifice of painting can approach.

For the rest, the phenomena of diffraction are not yet arranged systematically enough to be usefully discussed; some of them involving the resolution of the light, and others merely its intensification. My attention was first drawn to them near St. Laurent, on the Jura mountains, by the vivid reflection, (so it seemed), of the image of the sun from a particular point of a cloud in the west, after the sun itself was beneath the horizon: but in this image there were no prismatic colors, neither is the constantly seen metamorphosis of pine forests into silver filigree on ridges behind which the sun is rising or setting, accompanied with any prismatic hue; the trees become luminous, but not iridescent: on the other hand, in his great account of his ascent of Mont Blanc with Mr. Huxley, Professor Tyndall thus describes the sun's remarkable behavior on that occasion:—"As we attained the brow which forms the entrance to the Grand Plateau, he hung his disk upon a spike of rock to our left, and, surrounded by a glory of interference spectra of the most gorgeous colors, blazed down upon us." ('Glaciers of the Alps,' p. 76.)

Nothing irritates me more, myself, than having the color of my own descriptions of phenomena in anywise attributed by the reader to accidental states either of my mind or body;—but I cannot, for once, forbear at least the innocent question to Professor Tyndall, whether the extreme beauty of these 'interference spectra' may not have been partly owing to the extreme sobriety of the observer? no refreshment, it appears, having been attainable the night before at the Grands Mulets, except the beverage diluted with dirty snow, of which I have elsewhere quoted the Professor's pensive report,—"my memory of that tea is not pleasant."

16

'Either stationary or slow in motion, reflecting unresolved light.'

The rate of motion is of course not essentially connected with the method of illumination; their connection, in this instance, needs explanation of some points which could not be dealt with in the time of a single lecture.

It is before said, with reserve only, that "a cloud is where it is seen, and is not where it is not seen." But thirty years ago, in 'Modern Painters,' I pointed out (see the paragraph quoted in note 8th), the extreme difficulty of arriving at the cause of cloud outline, or explaining how, if we admitted at any given moment the atmospheric moisture to be generally diffused, it could be chilled by formal chills into formal clouds. How, for instance, in the upper cirri, a thousand little chills, alternating with a thousand little warmths, could stand still as a thousand little feathers.

But the first step to any elucidation of the matter is in the firmly fixing in our minds the difference between windless clouds, unaffected by any conceivable local accident, and windy clouds, affected by some change in their circumstances as they move.

In the sunset at Abbeville, represented in my first diagram, the air is absolutely calm at the ground surface, and the motion of its upper currents extremely slow. There is no local reason assignable for the presence of the cirri above, or of the thundercloud below. There is no conceivable cause either in the geology, or the moral character, of the two sides of the town of Abbeville, to explain why there should be decorative fresco on the sky over the southern suburb, and a muttering heap of gloom and danger over the northern. The electric cloud is as calm in motion as the harmless one; it changes its forms, indeed; but imperceptibly; and, so far as can be discerned, only at its own will is exalted, and with its own consent abased.

But in my second diagram are shown forms of vapor sustaining at every instant all kinds of varying local influences; beneath, fastened down by mountain attraction, above, flung afar by distracting winds; here, spread abroad into blanched sheets beneath the sunshine, and presently gathered into strands of coiled cordage in the shade. Their total existence is in metamorphosis, and their every aspect a surprise, or a deceit.

17

'Finely comminuted water or ice.'

My impression that these clouds were glacial was at once confirmed by a member of my audience, Dr. John Rae, in conversation after the lecture, in which he communicated to me the perfectly definite observations which he has had the kindness to set down with their dates for me, in the following letter:—