'Each adopts a chain of circumstances for the most part inapplicable in the case of the other; each avails himself of their common privilege of "daring everything to accomplish his end," not scrupling on some occasions to run counter, if necessary, even to matter of fact; for though most strictly bound to the observance of truth and probability, these are obviously very different from such as is required in _history_; his truth is the truth of _effect_, and his probability the perfect _harmony_ and _congruity_ of all the parts of his story, and their fitness to bring about the intended effect--that of striking the imagination, touching the passions, and developing in the most forcible manner the leading sentiment of the subject.' Much would naturally be expected from Opie when he came to treat of 'Chiaroscuro,' for it was one of his strong points. West said of him: 'He painted what he saw in the most masterly manner, and he varied little from it. _He saw nature in one point more distinctly and forcibly than any painter that ever lived._ The truth of colour as conveyed to the eye through the atmosphere, by which the distance of every object is ascertained, was never better expressed than by him.' And the following fine description of Opie's idea of this recondite branch of art accordingly seems to me worthy of the painter and writer: 'Light and shade must be allowed to be the creator of body and space. In addition to this, if properly managed, it contributes infinitely to expression and sentiment; it lulls by breadth and gentle gradation, strikes by contrast, and rouses by abrupt transition. All that is grave, impressive, awful, mysterious, sublime, or dreadful in nature, is really connected with IT. All poetic scenery, real or imaginary, "of forests and enchantments drear," where more is meant than is expressed; all the effects of solemn twilight and visionary obscurity that flings half an image on the aching sight; all the terrors of storm and the horrors of conflagration are indebted to IT for representation on canvas; IT is the medium of enchanting softness and repose in the works of some painters, and the vehicle by which others have risen to sublimity in spite of the want of almost every other excellence.' We now come to his last lecture, that on 'Colour,' and in it note this felicitous, nay poetic, passage: