OBJECT, in philosophy, something ap prehended, or presented to the mind, by sensation or by imagination.
Onreurginss of a telescope, or microscope, the glass placed at the end of the tube which is next the object.
To prove the goodness and regularity of an object-glass, 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 circumfer ence into six equal parts, pricking the points of. division through with a fine needle ; cover one side of the glass with this paper, and, exposing it to the sun, receive the rays through these six holes upon a plane ; then by moving the plane nearer to, or further from the glass, it will be found whether the six rays unite ex actly together at any distance from the glass ; if they do, it is a proof of the regu larity 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 object-glass, 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 con cave eye-glasses, without colouring or haziness. A circular object-glass is said
to be truly centered when the centre of its circumference falls exactly in the axis the glass ; and to be ill centered when it falls out of the axis. To prove whether object-glasses be well centered, 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 centered. 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 centered by fixing the tube, and observ ing, while the cell is unscrewed, whether the cross-hairs keep fixed upon the same lines of an object seen through the teles cope.