4. _The Process of Perception of Secondary Qualities._--The likenesses or representatives of objects by which they affect our senses were called by some writers _species_, or _sensible species_, a term which continued in use till the revival of science. It may be observed that the conception of these _species_ as films cast off from the object, and retaining its shape, was different, as we have seen, from the view which Aristotle took, though it has sometimes been called the Peripatetic doctrine[6\4]. We may add that the expression was latterly applied to express the supposition of an emanation of any kind, and implied little {300} more than that supposition of a Medium of which we are now speaking. Thus Bacon, after reviewing the phenomena of sound, says[7\4], 'Videntur motus soni fieri per _species spirituales_: ita enim loquendum donec certius quippiam inveniatur.' [Note 6\4: Brown, vol. ii. p. 98.] [Note 7\4: _Hist. Son. et Aud._ vol. ix. p. 87.] Though the fundamental principles of several sciences depend upon the assumption of a Medium of Perception, these principles do not at all depend upon any special view of the Process of our perceptions. The mechanism of that process is a curious subject of consideration; but it belongs to physiology, more properly than either to metaphysics, or to those branches of physics of which we are now speaking. The general nature of the process is the same for all the senses. The object affects the appropriate intermedium; the medium, through the proper organ, the eye, the ear, the nose, affects the nerves of the particular sense; and, by these, in some way, the sensation is conveyed to the mind, But to treat the _impression_ upon the nerves as the _act_ of sensation which we have to consider, would be to mistake our object, which is not the constitution of the human body, but of the human mind. It would be to mistake one link of the chain for the power which holds the end of the chain. No anatomical analysis of the corporeal conditions of vision, or hearing, or feeling warm, is necessary to the sciences of Optics, or Acoustics, or Thermotics.