The mind perceives the distance of objects, 1st, From the different con figurations of the eye, and the manner in which the rays strike the eye, and in which the image is impressed upon it. For the eye disposes itself differently, according to the different distances it is to, see ; viz. fpr remote objects the pupil is dilated, and the crystalline brought nearer the retina, and the whole eye is made more globous ; OR the contrary, for near objects, the pupil is contracted, the crystalline thrust forwards, and the eye lengthened. Again, the distance of vi sible objects is judged of by the angle the object makes ; from the distinct or confused representation of the objects ; and from the briskness or feebleness, or the rarity or density of the rays. To this it is owing, 1st, That objects which ap pear obscure or conftised, are judged to be more remote ; a principle which the painters make use of, to cause some of their figures to appear further distant than others on the same plane. 2d, To this it is likewise owing, that rooms whose walls are whitened, appear the smaller ; that fields covered with snow, or white flowers, appear less than when clothed with grass ; that mountains covered with snow, in the night time, appear the near er; and that opaque bodies appear the more remote in the twilight.
The magnitude of visible objects is known chiefly by the angle contained be tween two rays drawn from the two ex tremes of the object to the centre of the eye. An object appears so large as is the angle it subtends ; or bodies seen under a greater angle, appear greater ; and those under a ess angle, less, &c., Hence the same thing appears greater or less, as it is nearer the eye or further off. And this is callei the apparent magnitude.. But to judge of the real magnitude of an object, we must consider the distance : for since a near and a remote object may appear under equal 'angles, though the magnitudes be different, the distance must necessarily be estimated, because the magnitude is great or small according as the distance is. So that the real mag nitude is in the compound ratio of the distance and the apparent magnitude ; at least when the subtended angle, or appa fent magnitude is very small ; otherwise, the real magnitude will be in a ratio com pounded of the distance and the sine of the apparenenagnitude, nearly, or nearer still itstangeiA. Hence, objects seen under
the same angle, have their magnitudes in the same ratio as their distances. The chord of an arc of a circle appears of equal magnitude froth every point in the cir cumference, though one point be vastly `nearer than another. Or if the eye be 'fixed in any point in the circumference, and a right line be moved round so as its extremes be always in the periphery, it will appear of the same magnitude in every position. And the reason is, because the angle it subtends is always of the same niagnitu de. And hence also, -the eye being placed in any angle of a regular polygon, the.sides of it will all appear of equal magnitude ; Ire* all equal chords of a`cirele described about it. If the mag nitude 11 an object directly opposite to the eye be equal to its distance from the eye, the whole object will be distinctly seen, or taken in by the eye, but nothing more. And the nearer you approach an object, the less part you see of it. The least angle under which an ordinary ob ject becomes visible, is about one minute of 5 degree.
The figure of visible objects is estimat ed chiefly from our opinion of the situa tion of the several parts of the object. This opinion of the situation, &c. ena bles the mind to apprehend an external object under this or that figure, more 'justly than any similitude of the images . in the retina with the object can ; the images being often elliptical, oblong, &c. when the objects they exhibit to the mind are circles or squares, &c.
The laws of vision, with regard to the figures of visible objects, are, 1. That if the centre of the eye he exactly in the direction of a right line, the line will appear only as a point. 2. If the eye be placed in the direction of a surface, it will appear only as a line. 3. If a body be opposed directly towards the eye, so as only one plane of the surface can ra diate on it, the hotly will appear as a surface. 4. A remote arch, viewed by an eye in the same plane with it, will ap pear as a right line. 5. A sphere, view ed at a distance, appears a circle. 6. Angular figures, at a distance, appear round. 7. if the eye look obliquely on the centre of a regular figure, or a cir cle, the true figure will not be seen ; but the circle will appear oval, &c.