The most important part of the apparatus is however a pair of brass tools, one concave, and the other convex, made exactly to the curvature of the templates, and to fit each other as accurately as possible. The conoave tool is used as the grinder for correcting the curvature of the lenses, after they have been roughly figured in the concave shell. And the convex tool is employed for producing and maintaining the true form of the concave grinding tool itself, and also that of the polisher. The pair of brass tools are represented in section in fig. 1128.
The backs of the tools are provided with a screw exactly the same as an ordinary chuck, by which they may be fitted on the lathe mandrel, to be turned to the curvature of the templates, and by which they may also be attached to the top of a perpendicular post or pedestal, about three feet high, strongly fixed to the floor of the workshop, and carrying at the top an iron block having a vertical screw, exactly a copy of that upon the lathe mandrel.
The pair of brass tools having been turned to the curvature of the templates, they are next corrected by grinding them together; for this purpose the convex tool is fixed by its screw upon the perpendicular post, and the screw at the back of the concave tool is fitted with a wooden handle of a bulbous form, and sufficiently large to be grasped by the two hands.
The concave tool is placed upon the convex, and the two are rubbed together, first without any grinding powder, to denote by the parts brightened where they bear the hardest, as the manner in which they are ground together depends in some respects upon the nature of the general error to be corrected. Should the tools not fit each other tolerably well, the principal errors are reduced by turning until they agree nearly uniformly throughout their surfaces and touch about equally at the center and margins of the tools, the minute errors are then removed by grinding, which is usually done with emery and water, but as previously explained at page 1229, with respect to the grinding of flat tools for the parallel disks for sextants, Mr. Andrew Ross found that greater accuracy was obtained by using the emery dry. But whether wet or dry grinding be resorted to for the correction of the tools, the emery should be as uniformly distributed as possible, by rubbing it level with a piece of glass of corresponding curvature, and any excess of emery around the margin of the tool is wiped off, as there should be rather a deficiency than otherwise near the edges.
The concave tool is now placed upon the convex, and worked with a circular swinging stroke, somewhat as in rubbing the hand over the upper surface of a large ball, but instead of the motion being given by the arms alone, the body should at the same time be swung round, also in a circular path, so as to give a free bold stroke to the tool, but continually varied a little in extent and direction. Between every few strokes the operator moves a little way around the post, so as to continually change the position in which he stands, and gradually travel round the post. In every position he twists the upper tool partly round in his hands, so as by the combination of the various movements to bring the two surfaces in contact in every possible position, and rub them upon each other at all angles.
If either at the commencement, or during the process of grinding, the tools should be found to bear the hardest near the middle of the curve, the strokes are made short, and occasionally varied from the circular path to that of a narrow ellipsis, and the pressure is principally applied vertically; but if the tools bear the hardest near the edges of the curve, long bold circular strokes are taken, with the pressure principally sideways. In extreme cases the concave tool is fixed on the post, and the convex tool held in the hands is worked within it, with a swinging stroke, so as to grind the tools at the sides only; but as a general rule, the convex tool is fixed on the post in all cases, as the spherical figure may be more conveniently ground in this position.
The determining of the length and direction of the stroke, and also whether it should be circular or elliptical, are points that must be left principally to the judgment of the operator, guided in great measure by the sense of feeling; but, speaking generally, it may be said that large circular strokes increase the radius of curvature of the concave tool, from the margins being more acted upon than the center; while short elliptical strokes have the contrary effect. The curvature of the convex tool undergoes much less change, from the two modes of working, and therefore when it is desired to alter the curvature, the convex tool is first employed to alter the concave tool, and the convex is then fitted to it. The principal object aimed at is to make the tools of the true spherical figure, and to fit each other exactly, a small departure from the intended 'radius being in general less important than the correctness of the figure.
The glass for the lenses having been selected of suitable quality they are brought to the circular form with flat pliers called shanks, the jaws of which are made of soft iron that they may the more readily embed themselves upon the glass and take a firm hold; if the jaws were made of hardened steel they would be liable to slip. The pressure of the pliers applied near the edges of the glass causes it to crumble away in small fragments, and the process which is called shanking or nibbling is continued until the glasses are made circular, and of a little larger diameter than the finished size of the lenses.
They are next coated on one side with a layer of cement about half an inch thick to form a handle, by pouring the melted cement from a ladle upon the glass in small quantities, as much as will lay on the glass without running off, and as soon as it is set, a further supply is added until the cement forms a hemispherical mass, sufficiently thick to be readily grasped in the fingers.