OBJECT

, something presented to the mind, by sensation, or by imagination. Or something that affects us by its presence, that affects the eye, ear, or some other of the organs of sense.|

The objects of the eye, or vision, are painted on the retina; though not there erect, but inverted, according to the laws of optics. This is easily shewn from Des Cartes's experiment, of laying bare the vitreous humour on the back part of the eye, and putting over it a bit of white paper, or the skin of an egg, and then placing the fore part of the eye to the hole of a darkened room. By this means there is obtained a pretty landscape of the external objects, painted invertedly on the back of the eye. In this case, how the Objects thus painted invertedly should be seen erect, is matter of controversy.

Object is also used for the subject, or matter of an art or science; being that about which it is employed or concerned.

Object-Glass, of a telescope or microscope, is the glass placed at the end of the tube which is next or towards the Object to be viewed.

To prove the goodness and regularity of an Objectglass; on a paper describe two concentric circles, the one having its diameter the same with the breadth of the Object-glass, and the other half that diameter; divide the smaller circumference into 6 equal parts, pricking the points of division through with a sine needle; cover one side of the glass with this paper, and, exposing it to the sun, receive the rays through these 6 holes upon a plane; then by moving the plane nearer to or farther from the glass, it will be found whether the six rays unite exactly together at any distance from the glass; if they do, it is a proof of the regularity and just form of the glass; and the said distance is also the focal distance of the glass.

A good way of proving the excellency of an Objectglass, is by placing it in a tube, and trying it with small eye-glasses, at several distant objects; for that Object-glass is always the best, which represents objects the brightest and most distinct, and which bears the greatest aperture, and the most convex and concave eyeglasses, without colouring or haziness.

A circular Object-glass is said to be truly centred, when the centre of its circumference falls exactly in the axis of the glass; and to be ill centred, when it falls out of the axis.

To prove whether Object-gla<*>es be well centred, hold the glass at a due distance from the eye, and observe the two reflected images of a candle, varying the distance till the two images unite, which is the true centre point: then if this fall in the middle, or central point of the glass, it is known to be truly centred.

As Object-glasses are commonly included in cells that screw upon the end of the tube of a telescope, it may be proved whether they be well centred, by fixing the tube, and observing while the cell is unscrewed, whether the cross-hairs keep fixed upon the same lines of an object seen through the telescope.

For various methods of finding the true centre of an Object-glass, see Smith's Optics, book 3, chap. 3; also the Philos. Trans. vol. 48, pa. 177.

OBJECTIVE Line, in Perspective, is any line drawr on the geometrical plane, whose representation is sought for in a draught or picture.

Objective Plane, in Perspective, is any plane situated in the horizontal plane, whose perspective representation is required.