Such mist, however, only becomes prismatic as common rain does, when the sun is behind the spectator, while prismatic clouds are, on the contrary, always between the spectator and the sun. The main reason, however, why I can tell you nothing yet about these colours of diffraction or interference, is that, whenever I try to find anything firm for you to depend on, I am stopped by the quite frightful inaccuracy of the scientific people's terms, which is the consequence of their always trying to write mixed Latin and English, so losing the grace of the one and the sense of the other. And, in this point of the diffraction of light I am stopped dead by their confusion of idea also, in using the words undulation and vibration as synonyms. "When," says Professor Tyndall, "you are told that the atoms of the sun vibrate at different rates, and produce waves of different sizes,-your experience of water-waves will enable you to form a tolerably clear notion of what is meant." Tolerably clear'!-your toleration must be considerable, then. Do you suppose a water-wave is like a harp-string? Vibration is the movement of a body in a state of tension,undulation, that of a body absolutely lax. In vibration, not an atom of the body changes its place in relation to another, -in undulation, not an atom of the body remains in the same place with regard to another. In vibration, every particle of the body ignores gravitation, or defies it,-in undulation, every particle of the body is slavishly submitted to it. In undulation, not one wave is like another; in vibration, every pulse is alike. And of undulation itself, there are all manner of visible conditions, which are not true conditions. A flag ripples in the wind, but it does not undulate as the sea does, -for in the sea, the water is taken from the trough to put on to the ridge, but in the flag, though the motion is progressive, the bits of bunting keep their place. You see a field of corn undulating as if it was water,-it is different from the flag, for the ears of corn bow out of their places and return to them, --and yet, it is no more like the undulation of the sea, than the shaking of an aspen leaf in a storm, or the lowering of the lances in a battle. And the best of the jest is, that after mixing up these two notions in their heads inextricably, the scientific people apply both when neither will fit; and when all undulation known to us presumes weight, and all vibration, impact,-the undulating theory of light is proposed to you concerning a medium which you can neither weigh nor touch! All communicable vibration-of course I mean-and in dead matter: You may fall a shivering on your own account, if you like, but you can't get a billiard-ball to fall a shivering on its own account. 18 Yet observe that in thus signalizing the inaccuracy of the terms in which they are taught, I neither accept, nor assail, the conclusions respecting the oscillatory states of light, heat, and sound, which have resulted from the postulate of an elastic, though impalpable and imponderable ether, possessing the elasticity of air. This only I desire you to mark with attention, that both light and sound are sensations of the animal frame, which remain, and must remain, wholly inexplicable, whatever manner of force, pulse, or palpitation may be instrumental in producing them: nor does any such force become light or sound, except in its rencontre with an animal. The leaf hears no murmur in the wind to which it wavers on the branches, nor can the clay discern the vibration by which it is thrilled into a ruby. The Eye and the Ear are the creators alike of the ray and the tone; and the conclusion follows logically from the right conception of their living power,"He that planted the Ear, shall He not hear? He that formed the Eye, shall not He see?" For security, therefore, and simplicity of definition of light, you will find no possibility of advancing beyond Plato's "the power that through the eye manifests colour," but on that definition, you will find, alike by Plato and all great subsequent thinkers, a moral Science of Light founded, far and away more important to you than all the physical laws ever learned by vitreous revelation. Concerning which I will refer you to the sixth lecture which I gave at Oxford in 1872, or the relation of Art to the Science of Light (Eagle's Nest,' p. 75), reading now only the sentence introducing its subject: "The Fiat lux' of creation is therefore, in the deep sense, 'fiat anima,' and is as much, when you understand it, the ordering of Intelligence as the ordering of Vision. It is the appointment of change of what had been else only a mechanical effluence from things unseen to things unseeing,-from Stars, that did not shine, to Earth, that did not perceive,-the change, I say, of that blind vibration into the glory of the Sun and Moon for human eyes: so making possible the communication out of the unfathomable truth of that portion of truth which is good for us, and animating to us, and is set to rule over the day and over the night of our joy and our sorrow." Returning now to our subject at the point from which I permitted myself, I trust not without your pardon, to diverge; you may incidentally, but carefully, observe, that the effect of such a sky as that represented in the second diagram, so far as it can be abstracted or conveyed by painting at all, implies the total absence of any pervading warmth of tint, such as artists usually call 'tone.' Every tint must be the purest possible, and above all the white. Partly, lest you should think, from my treatment of these two phases of effect, that I am insensible to the quality of tone,-and partly to complete the representation of states of weather undefiled by plaguecloud, yet capable of the most solemn dignity in saddening colour, I show you, Diagram 3, the record of an autumn twilight of the year 1845,-sketched while I was changing horses between Verona and Brescia. The distant sky in this drawing is in the glowing calm which is always taken by the great Italian painters for the background of their sacred pictures; a broad field of cloud is advancing upon it overhead, and meeting others enlarging in the distance; these are rain-clouds, which will certainly close over the clear sky, and bring on rain before midnight but there is no power in them to pollute the sky beyond and above them: they do not darken the air, nor defile it, nor in any way mingle with it; their edges are burnished by the sun like the edges of golden shields, and their advancing march is as deliberate and majestic as the facing of the twilight itself into a darkness full of stars. These three instances are all I have time to give of the former conditions of serene weather, and of non-electric raincloud. But I must yet, to complete the sequence of my subject, show you one example of a good, old-fashioned, healthy, and mighty, storm. In Diagram 4, Mr. Severn has beautifully enlarged my sketch of a July thunder-cloud of the year 1858, on the Alps of the Val d'Aosta, seen from Turin, that is to say, some twenty-five or thirty miles distant. You see that no mistake is possible here about what is good weather and what bad, or which is cloud and which is sky; but I show you this sketch especially to give you the scale of heights for such clouds in the atmosphere. These thunder cumuli entirely hide the higher Alps. It does not, however, follow that they have buried them, for most of their own aspect of height is owing to the approach of their nearer masses; but at all events, you have cumulus there rising from its base, at about three thousand feet above the plain, to a good ten thousand in the air. White cirri, in reality parallel, but by perspective radiating, catch the sunshine above, at a height of from fifteen to twenty thousand feet; but the storm on the mountains gathers itself into a full mile's depth of massy cloud,-every fold of it involved with thunder, but every form of it, every action, every colour, magnificent :-doing its mighty work in its own hour and its own dominion, nor snatching from you for an instant, nor defiling with a stain, the abiding blue of the transcendent sky, or the fretted silver of its passionless clouds. We so rarely now see cumulus cloud of this grand kind, that I will yet delay you by reading the description of its nearer aspect, in the 113th page of 'Eagle's Nest.' "The rain which flooded our fields the Sunday before last, was followed, as you will remember, by bright days, of which Tuesday the 20th (February, 1872) was, in London, notable for the splendour, towards the afternoon, of its white cumulus clouds. There has been so much black east wind lately, and so much fog and artificial gloom, besides, that I find it is actually some two years since I last saw a noble cumulus cloud under full light. I chanced to be standing under the Victoria Tower at Westminster, when the largest mass of them floated past, that day, from the north-west; and I was more impressed then ever yet by the awfulness of the cloudform, and its unaccountableness, in the present state of our knowledge. The Victoria Tower, seen against it, had no magnitude it was like looking at Mont Blanc over a lamppost. The domes of cloud-snow were heaped as definitely: their broken flanks were as grey and firm as rocks, and the whole mountain, of a compass and height in heaven which only became more and more inconceivable as the eye strove to ascend it, was passing behind the tower with a steady march, whose swiftness must in reality have been that of a tempest yet, along all the ravines of vapour, precipice kept pace with precipice, and not one thrust another. "What is it that hews them out? Why is the blue sky pure there, the cloud solid here; and edged like marble and why does the state of the blue sky pass into the state of cloud, in that calm advance? "It is true that you can more or less imitate the forms of cloud with explosive vapour or steam; but the steam melts instantly, and the explosive vapour dissipates itself. The cloud, of perfect form, proceeds unchanged. It is not an explosion, but an enduring and advancing presence. The more you think of it, the less explicable it will become to you." Thus far then of clouds that were once familiar; now at last, entering on my immediate subject, I shall best introduce it to you by reading an entry in my diary which gives progressive description of the most gentle aspect of the modern plague-cloud. "BOLTON ABBEY, 4th July, 1875. "Half-past eight, morning; the first bright morning for the last fortnight. "At half-past five it was entirely clear, and entirely calm; the moorlands glowing, and the Wharfe glittering in sacred light, and even the thin-stemmed field-flowers quiet as stars, in the peace in which 'All trees and simples, great and small, That balmy leaf do bear, Than they were painted on a wall, |