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 color," 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, on the relation of Art to the Science of Light ('The Eagle's Nest'), 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 plague-cloud, yet capable of the most solemn dignity in saddening color, 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 fading of the twilight itself into a darkness full of stars.