Form, in its widest meaning, the visible universe that envelopes our senses, and its counterpart the invisible one that agitates our mind with visions bred on sense by fancy, are the element and the realm of invention; it discovers, selects, combines the _possible_, the _probable_, the _known_, in a mode that strikes with an air of truth and novelty, at once. Possible strictly means an effect derived from a cause, a body composed of materials, a coalition of forms, whose union or co-agency imply in themselves no absurdity, no contradiction: applied to our art, it takes a wider latitude; it means the representation of effects derived from causes, or forms compounded from materials, heterogeneous and incompatible among themselves, but rendered so plausible to our senses, that the transition of one part to another seems to be accounted for by an air of organization, and the eye glides imperceptibly or with satisfaction from one to the other and over the whole: that this was the condition on which, and the limits within which alone the ancients permitted invention to represent what was strictly speaking impossible, we may with plausibility surmise from the picture of Zeuxis, described by Lucian in the memoir to which he has prefixed that painter's name, who was probably one of the first adventurers in this species of imagery.--Zeuxis had painted a family of centaurs; the dam a beautiful female to the middle, with the lower parts gradually sliding into the most exquisite forms of a young Thessalian mare, half reclined in playful repose and gently pawing the velvet ground, offered her human nipple to one infant centaur, whilst another greedily sucked the ferine udder below, but both with their eyes turned up to a lion-whelp held over them by the male centaur their father, rising above the hillock on which the female reclined, a grim feature, but whose ferocity was somewhat tempered by a smile.