§ XVII. Within its due limits, however, here is one branch of science which the artist may pursue; and, within limits still more strict, another also, namely, the science of the appearances of things as they have been ascertained and registered by his fellow-men. For no day passes but some visible fact is pointed out to us by others, which, without their help, we should not have noticed; and the accumulation and generalization of visible facts have formed, in the succession of ages, the sciences of light and shade, and perspective, linear and aerial: so that the artist is now at once put in possession of certain truths respecting the appearances of things, which, so pointed out to him, any man may in a few days understand and acknowledge; but which, without aid, he could not probably discover in his lifetime. I say, probably could not, because the time which the history of art shows us to have been actually occupied in the discovery and systematization of such truth, is no measure of the time _necessary_ for such discovery. The lengthened period which elapsed between the earliest and the perfect developement of the science of light (if I may so call it) was not occupied in the actual effort to ascertain its laws, but in _acquiring the disposition to make that effort_. It did not take five centuries to find out the appearance of natural objects; but it took five centuries to make people care about representing them. An artist of the twelfth century did not desire to represent nature. His work was symbolical and ornamental. So long as it was intelligible and lovely, he had no care to make it like nature. As, for instance, when an old painter represented the glory round a saint's head by a burnished plate of pure gold, he had no intention of imitating an effect of light. He meant to tell the spectator that the figure so decorated was a saint, and to produce splendor of effect by the golden circle. It was no matter to him what light was like. So soon as it entered into his intention to represent the appearance of light, he was not long in discovering the natural facts necessary for his purpose.