retina, or of our eyes in following the objects. After some time, we learn to make allowance for the line of direction, our own motions, &c. If we fail to make the due allowance through associated circumstances of any kind, we must, in consequence of this, make a disproportionate estimate of motion, or place it in an undue object.
We judge of the figure or shape of bodies, chiefly by the variations of light and shade; and our associations taken thence are so strong, as that we are easily imposed upon by a just imitation of the light and shades belonging to each shape and figure, in their several situations with respect to the quarter from which the illumination proceeds.
It is from the associations, considered under this proposition, and particularly in the last paragraph, that painting conveys such exact ideas of shapes, figures, magnitudes, and distances, and the camera obscura of motions also, by means of impressions that proceed from a plane surface.
The position of objects is judged of entirely by the part of the retina on which the rays fall, if we be in an erect posture ourselves. If we be not, we allow for our deviation from it, or make a reference to something judged to be in an erect posture. If we fail in these, errors concerning the position of visible objects must happen. Our calling bodies erect, when the rays proceeding from their tops fall upon the lower parts of the retina, and vice versâ, is merely from an association of the same kind with those by which the senses of other words are determined.
Those who are disposed to examine the subjects of this and the following proposition with accuracy, may see a large variety of proper instances well explained by Dr. Smith, and Dr. Jurin, in Dr. Smith’s Optics. These gentlemen insist chiefly on optical considerations; but they every where admit the prevalence of association, though it is not always to their purpose to take express notice of it.
I will just remind the reader, that in all the cases of magnitude, distance, motion, figure, and position, the visible idea is so much more vivid and ready than the tangible one, as to prevail over it, notwithstanding that our information from feeling is more precise than that from sight, and the test of its truth. However, if we could suppose a person to be endued with the senses of seeing and hearing, and yet to be destitute of that of feeling, and of the power of moving himself, he might have all the words expressing distances, magnitudes, &c. so much, and so properly, associated with the visible appearances of these, as that, by passing over his ear, they would raise up all the same trains of visible ideas, as in us.
