_Secondary Qualities are not_ extended _but_ intensive: _their effects are not augmented by addition of parts, but by increased operation of the medium. Hence they are not measured directly, but by_ scales; _not by_ units, _but by_ degrees. (IV. 4.) LX. _In the Scales of Secondary Qualities, it is a condition_ (_in order that the scale may be complete,_) _that every example of the quality must either_ agree _with one of the degrees of the Scale, or lie between two_ contiguous _degrees._ (IV. 4.) LXI. _We perceive_ by means of _a medium and_ by means of _impressions on the nerves: but we do not_ (_by our senses_) _perceive either the medium or the impressions on the nerves._ (IV. 1.) LXII. _The_ Prerogatives of the Sight _are, that by this sense we necessarily and immediately apprehend the_ position _of its objects: and that from visible circumstances, we_ infer _the_ distance _of objects from us, so readily that we seem to perceive and not to infer._ (IV. 2.) {17} LXIII. _The_ Prerogatives of the Hearing _are, that by this sense we perceive relations perfectly precise and definite between two notes, namely,_ Musical Intervals (_as an_ Octave, _a_ Fifth); _and that when two notes are perceived together, they are comprehended as distinct,_ (_a_ Chord,) _and as having a certain relation,_ (Concord _or_ Discord.) (IV. 2.) LXIV. _The Sight cannot decompose a compound colour into simple colours, or distinguish a compound from a simple colour. The Hearing cannot directly perceive the place, still less the distance, of its objects: we infer these obscurely and vaguely from audible circumstances._ (IV. 2.) LXV. _The_ First Paradox of Vision _is, that we see objects_ upright, _though the images on the retina are_ inverted. _The solution is, that we do not see the image on the retina at all, we only see by means of it._ (IV. 2.) LXVI. _The_ Second Paradox of Vision _is, that we see objects_ single, _though there are two images on the retinas, one in each eye. The explanation is, that it is a Law of Vision that we see_ (_small or distant_) _objects single, when their images fall on_ corresponding points _of the two retinas._ (IV. 2.) LXVII. _The law of single vision for_ near _objects is this:--When the two images in the two eyes are situated, part for part, nearly but not exactly, upon corresponding points, the object is apprehended as single and solid if the two objects are such as would be produced by a single solid object seen by the eyes separately._ (IV. 2.) LXVIII. _The ultimate object of each of the Secondary Mechanical Sciences is, to determine the nature and laws of the processes_ {18} _by which the impression of the Secondary Quality treated of is conveyed: but before we discover the cause, it may be necessary to determine the_ laws _of the phenomena; and for this purpose a_ Measure _or_ Scale _of each quality is necessary._ (IV. 4.) LXIX.