The bustle of a croud is not ill-adapted to the pencil: but the management of it requires great artifice. The whole must be massed together, and considered as one body. ¶ I mean not to have the whole body so agglomerated, as to consist of no detached groups: but to have these groups […] appear to belong to one whole, by the artifice of composition, and the effect of light.
Source: wiktionary