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.
Secondary qualities are measured by means of such effects as can be estimated in number or space. (iv. 4.)