"The Hay Wain" established his fame; but Constable was not the man to sit down under success and repeat his triumphs in one particular method. In the interval between the painting of "The Hay Wain" and its exhibition in Paris, he produced "Salisbury from the Bishop's Garden," now in the South Kensington collection, wherein he attempted to represent the glitter of sunlight by spots of pure pigment which his friends called "Constable's snow." To us, accustomed to modern pictures of sunlight, the "spots and scumbles of pure pigment" in "Salisbury Cathedral from the Bishop's Garden" are hardly noticeable, but in 1823 they were an innovation, although not altogether a new discovery. Pinturicchio, in his frescoes in the library of Siena Cathedral, experimented in pointillism, and you may trace it, too, in some of the pictures by Vermeer of Delft. "Salisbury from the Bishop's Garden" gave Constable considerable trouble. He was ill and his children were ill. "What with anxiety, watching, nursing, and my own indisposition, I have not see the face of my easel since Christmas, and it is not the least of my troubles that the good Bishop's picture is not yet fit to be seen." Later he describes "Salisbury from the Bishop's Garden" as "the most difficult subject in landscape I ever had upon my easel," adding that it "looks uncommonly well," and that "I have not flinched at the windows, buttresses, etc., but I have still kept to my grand organ colour, and have, as usual, made my escape in the evanescence of the Chiaroscuro." "The Lock," another of his well-known pictures, was purchased from the Royal Academy Exhibition of 1824 by Mr Morrison for "one hundred and fifty guineas, including the frame." The superb oil sketch for "The Lock" was sold at Christie's in 1901 for nineteen hundred guineas. It is an upright picture of sunshine and gusty wind, and represents a lock-keeper opening the gates for the passage of a boat. "My 'Lock'" wrote Constable to Fisher, "is liked at the Academy, and indeed it forms a decided feature, and its light cannot be put out, because it is the light of Nature, the mother of all that is valuable in poetry, painting, or anything else where an appeal to the soul is required.... But my execution annoys most of them, and all the scholastic ones. Perhaps the sacrifices I make for lightness and brightness are too great, but these things are the essence of landscape, and my extreme is better than white-lead and oil, and dado painting." Probably no other landscape painter has expressed the intention of his art as clearly in writing as with his brushes. Light! The light of Nature! The mother of all that is valuable in painting! That was Constable's secret--the knowledge of light, a secret that was hidden from the eyes of worthy Sir George Beaumont.