By following those instructions amateurs who have a small lathe not adapted for screw-cutting, and who are not adepts in the use of chasers, can make the brass cells and similar work for microscopes, telescopes, etc. The apparatus here described will turn and cut the threads without displacement, thus ensuring perfect centreing, without which the best lenses will give unsatisfactory results. To hold the cells, etc., use boxwood chucks fixed ou iron face-plates. A hole is drilled truly in the centre of the chuck while in the tin he. Into this hole fits a turned iron or steel mandrel of the shape shown at Figs. 1 and 2. The part 0 should be a tight Working fit in the boxwood chuck. The poppet end of the mandrel has a thread out on it of a pitch suitable for optical work. Fig. 3 shows the complete mandrel and tool-rest. The hole B (Fig. 2) is tapped to receive the screw that regulates the cut of the tool, while into thep hole A (Fig. 2) slides the guide; and the set-screw E (Fig. 3) takes up any shake in the rest. To complete the tool-rest, pieces v \ Fig. 3) to carry the tool and G for the handle end will be required. The ordinary poppet must be discarded; in its place use a wrought- or cast-iron poppet, made as shown in Figs. 4, 5, and 6. The hole 31 (Figs, 4 and 5) receives the bush I (Fig. 6), which is drilled and tapped to suit the screwed end of the mandrel M. J and K are nuts, and L is a handle made fast to the mandrel; it actuates the cut of the tool longitudinally. N (Figs. 4, 5, and 6) is the hole used for bolting down the poppet. When facing, boring, or turning a cell, etc., the nut J is released and the nut K is jammed; then I can revolve, the cut being regulated by the handle L. "When thread-cutting, the lathe spindle carrying the chuck must be fixed so that it will not turn; then the nut J is jammed tight, thus fixing I, the cut being actuated by the handle L. The thread may be started at any point desired. Fig. 7 shows the complete apparatus, with letter references as before. If the use of a lathe is not to be had, the apparatus will still be of use, but in that case all operations of turning and screw-cutting must be managed by the handle L, while the work remains at rest. The sketches are not to scale, and the apparatus must be made to suit the lathe in use.

Making Cells for Optical Work.