Mr. C. Varley has described Trans. Society of Arts; vol. xlix.) a the for grinding and polishing lenses in which instead of the lower )ols being mounted on a fixed post they are mounted on a revolving xis placed vertically. Mr. Grubb, of Dublin, has an apparatus for figur ig and polishing lenses, which is said to be very successful. For very ccurate work, the arrangement shown in fig. 12 may be adopted. After grinding in the metal asin ( 11), the lens isattached to the lower end f a vertical rod, of which the upper end tenni atea in a steel ball, working in a cup and fitting ccurately, so that every point of the surface of he lens may move in a spherical surface concen ric with the steel ball. The rod is grasped by a ,00llen holder, to prevent the heat of the hand rum elongating the rod. Below the lens is a mall polisher of pitch, spread on brass and overed with a fine polishing powder mixed rith water. The polisher admits of nice djustment by means of a screw. The rod is corked to and fro as well as circularly, and the ens gradually acquires a perfectly spherical .nd polished surface, the radius of which can its adjusted by lengthening or shortening the od. In manufactories where large quantities if common lenses are ground and polished, a lumber are arranged on a convex tool, such as 13, or 21 around a central lens, forming slat is called a block of lenses. Lenses of nedium quality and size are generally ground ,rue and polished seven at a time.
In forming the object glasses of achromatic telescopes, it is necessary to measure accurately le radii of curvature of the lenses, which are first tried experimentally, and are afterwards made as nearly as possible to the radii obtained oy calculation. In order to measure the cur
vature of the grinding tools, Mr. Ross invented in instrument called a *Aerometer, a descrip tion of which will be found in the ' Trans. society of Arta,' (vol. liii.) The edges of lenses are made true by grind ing in a lathe by means of a piece of brass supplied with emery and water ; but in setting the lenses in the lathe for this purpose, it is cemented upon a chuck, and before the cement has set, the lathe is set spinning, and the reflection of a fixed object, such as a candle flame or a window bar is watched, and the lens is adjusted until the image appears to be quite stationary, notwithstanding the revolution of tho lens. This shows that the axis of the lens and that of the mandril of the lathe coincide and consequently that the lens, when its edge is ground cir cular, and put in the tube of the instrument, will coincide with the axis of such tube.
The centering of the lenses of an achromatic telescope, so that all the centres of curvature of their surfaces shall lie in one straight line, and that coincide with the common axis of the telescope and its eye piece, is an adjustment of considerable delicacy. Wollaston's method of doing this is described in the ' Philosophical Transactions' for 1822, and depends upon the circumstance that the various images of a candle, of which there are fifteen for a triple object glass, are, if the lenses be exactly centered, all in a right line directed from tho candle.