This is a rough classification, for an opera does not necessarily compare with a square rood of paint or a novel of Tolstoyan length; indeed, on the volumetric basis, an opera may have less bulk than a sonnet. Group A. (Volumetric). By _concentration_ we mean the quality of conveying a great deal within a small space. It follows that _concentration_ is in inverse ratio to area, though it does not follow that area is in inverse ratio to _concentration_. While _Anna Karenin_ is an enormous novel it is as concentrated as the sonnet of d'Arvers; on the other hand, Francis Thompson's _Arab Love Song_ is more concentrated than the complete works of Mrs Barclay; while any Rubens is more concentrated than a modern miniature, an intaglio may be more concentrated than twenty square yards of Delacroix. We nullify areas, therefore, and must lay down that the test of concentration is the effect: if the painter realises that the author has felt all he wrote, if the writer sees that every line was necessary, then both can be sure that they are respectively in presence of concentrated works. Likewise with _relief_. A bas-relief may have none. A fresco may. _Relief_ then is a matter of contrast, as is shown especially in the mosaics of Taj Mahal; but its nature is easily seen if we compare prose with paint:-- 'He stood at the edge of the sea while the waves crept towards him, nearer and nearer, sinuously flowing and ebbing, but ever nearer. Ever.' I give this as an instance, not as a fragment of literature. The lonely 'ever' gives _relief_ to the sentence of twenty-four words if we assume that another long sentence follows. (If no sentence follows, 'ever' is no longer _relief_ but _culmination_, see Group C.) The painter renders the same effect by a more vivid line of foam in the middle distance, the musician by interposing a treble motif between basses. Thus, if we find variety of sentence, variety of tone, we have _relief_. _Density_ and _Depth_ need not detain us long. Flaubert, the Psalms, Jacob Epstein's _Oscar Wilde_, the Eroica and Velasquez all give the sensation we call by those names; we mean by them that the work contains a suggestion of something behind. Atmospheric quality, then, together with thought withdrawn, echo unheard and space unlimned, are the bases on which the two terms rest. The suggestion that this 'behind' exists is of course essential, for we must not conclude that where there is nothing to be seen there is something to be guessed: there must be no guessing, but if a feeling of reserve is created then _density_ and _depth_ exist.