In the Preface to the first edition of Modern Painters I Ruskin explained that his aims had changed as the work developed:
But, as point after point presented itself for demonstration, I found myself compelled to amplify what was at first a letter to the Editor of a Review, into something very like a treatise on art.... I now scarcely know whether I should announce it as an Essay on Landscape Painting, and apologize for its frequent reference to the works of a particular master; or, announcing it as a critique on particular works, apologize for its lengthy discussion of general principles. ( MP I, 1843:vii-viii)
(See long gestation period of Modern Painters I, Ruskin's later views on Modern Painters I, letter to Blackwood's (1836), Ruskin and Turner and Ruskin and aesthetics.)