127. And it is precisely in this passive and naïve simplicity that art becomes, not only greatest in herself, but most useful to science. If she _knew_ anything of what she was representing, she would exhibit that partial knowledge with complacency; and miss the points beside it, and beyond it. Two painters draw the same mountain; the one has got unluckily into his head some curiosity about glacier marking; and the other has a theory of cleavage. The one will scratch his mountain all over;--the other split it to pieces; and both drawings will be equally useless for the purposes of honest science. 128. Any of you who chance to know my books cannot but be surprised at my saying these things; for, of all writers on art, I suppose there is no one who appeals so often as I do to physical science. But observe, I appeal as a critic of art, never as a master of it. Turner made drawings of mountains and clouds which the public said were absurd. I said, on the contrary, they were the only true drawings of mountains and clouds ever made yet: and I proved this to be so, as only it could be proved, by steady test of physical science: but Turner had drawn his mountains rightly, long before their structure was known to any geologist in Europe; and has painted perfectly truths of anatomy in clouds which I challenge any meteorologist in Europe to explain at this day. 129. And indeed I was obliged to leave "Modern Painters" incomplete, or, rather, as a mere sketch of intention, in analysis of the forms of cloud and wave, because I had not scientific data enough to appeal to. Just reflect for an instant how absolutely whatever has been done in art to represent these most familiar, yet most spectral forms of cloud--utterly inorganic, yet, by spiritual ordinance, in their kindness fair, and in their anger frightful,--how all that has yet been done to represent them, from the undulating bands of blue and white which give to heraldry its nebule bearing, to the finished and deceptive skies of Turner, has been done without one syllable of help from the lips of science.[F] [F] Rubens' rainbow, in the Loan Exhibition this year, was of dull blue, _darker_ than the sky, in a scene lighted from the side of the rainbow. Rubens is not to be blamed for ignorance of optics, but for never having so much as looked at a rainbow carefully: and I do not believe that my friend Mr. Alfred Hunt, whose study of rainbow, in the rooms of the Water Colour Society last year, was unrivalled, for vividness and truth, by any I know, learned how to paint it by studying optics.