This was the fulfilment of his advice to young artists; and so far as young artists worked in this way, for purposes of study, he encouraged them. But he did not fail to point out that this was not all that could be required of them. Even such a work as Brett's "Val d'Aosta," marvellous as it was in observation and finish, was only the beginning of a new era, not its consummation. It was not the painting of detail that could make a great artist; but the knowledge of it, and the masterly use of such knowledge. A great landscapist would know the facts and effects of nature, just as Tintoret knew the form of the human figure; and he would treat them with the same freedom, as the means of expressing great ideas, of affording by the imagination noble grounds for noble emotion, which, as Ruskin had been writing at Vevey in 1854, was poetry. Meanwhile the public and the critic ought to become familiar with the aspects of nature, in order to recognise the difference between the true poetry of painting, and the mere empty sentimentalism which was only the rant and bombast of landscape art. With such feelings as these he wrote the third and fourth volumes of "Modern Painters," (published respectively January 15 and April 14, 1856). The work was afterwards interrupted only by a recurrence of his old cough, in the exceptionally cold summer of 1855. He went down to Tunbridge Wells, where his cousin, William Richardson of Perth, was practising as a doctor; it was not long before the cough gave way to treatment, and he was as busy as ever. About October of that year he wrote to Mrs. Carlyle as follows, in a letter printed by Professor C.E. Norton, conveniently summing up his year: "Not that I have not been busy--and very busy, too. I have written, since May, good six hundred pages, had them rewritten, cut up, corrected, and got fairly ready for press--and am going to press with the first of them on Gunpowder Plot day, with a great hope of disturbing the Public Peace in various directions. Also, I have prepared above thirty drawings for engravers this year, retouched the engravings (generally the worst part of the business), and etched some on steel myself. In the course of the six hundred pages I have had to make various remarks on German Metaphysics, on Poetry, Political Economy, Cookery, Music, Geology, Dress, Agriculture, Horticulture, and Navigation,[6] all of which subjects I have had to 'read up' accordingly, and this takes time. Moreover, I have had my class of workmen out sketching every week in the fields during the summer; and have been studying Spanish proverbs with my father's partner, who came over from Spain to see the Great Exhibition. I have also designed and drawn a window for the Museum at Oxford; and have every now and then had to look over a parcel of five or six new designs for fronts and backs to the said Museum.