The _pocket apparatus_ is similar to one originally made and used by Sir William J. Herschel (see Plate 3, Fig. 4, in which the roller and its bearings are drawn of the same size as those I use). A small cylinder of hard wood, or of brass tube, say 1-3/4 inch long, and 1/2 or 3/4 inch in diameter, has a pin firmly driven into each end to serve as an axle. A piece of tightly-fitting india-rubber tubing is drawn over the cylinder. The cylinder, thus coated with a soft smooth compressible material, turns on its axle in two brackets, each secured by screws, as shown in Plate 2, Fig. 4, to a board (say 6 x 2-1/2 x 1/4 inch) that serves as handle. This makes a very fair and durable roller; it can be used in the heat and damp of the tropics, and is none the worse for a wetting, but it is by no means so good for delicate work as a cylinder covered with roller composition. These are not at all difficult to make; I have cast them for myself. The mould is a piece of brass tube, polished inside. A thick disc, with a central hole for the lower pin of the cylinder, fits smoothly into the lower end of the mould, and a ring with a thin bar across it, fits over the other end, the upper pin of the cylinder entering a hole in the middle of the bar; thus the cylinder is firmly held in the right position. After slightly oiling the inside of the mould, warming it, inserting the disc and cylinder, and fitting on the ring, the melted composition is poured in on either side of the bar. As it contracts on cooling, rather more must be poured in than at first appears necessary. Finally the roller is pushed out of the mould by a wooden ramrod, applied to the bottom of the disc. The composition must be melted like glue, in a vessel surrounded by hot water, which should never be allowed to boil; otherwise it will be spoilt. Harrild's best composition is more than twice the cost of that ordinarily used, and is expensive for large rollers, but for these miniature ones the cost is unimportant. The mould with which my first roller was made, was an old pewter squirt with the nozzle cut off; its piston served the double purpose of disc and ramrod.