J. E. Paynter
A microtome is an appliance used for cutting very thin sections of the stems, leaves, etc., of plants, for examination under the microscope. One of the most common methods adopted by those who do not passess a microtome is to hold the stem or other object between two pieces of pith or potato, and then take off very fine parings of the potato, and, at the same time, of the object embedded therein, with a razor. This is not an ideal method, as it is difficult to obtain very thin sections; there is also an element of danger in the operation, seeing that the razor used for cutting has to be very sharp.
Some mechanical aid. therefore, is necessary if really satisfactory work is to be produced. There are many expensive and elaborate patterns of microtomes made for this purpose, but the one to be described will serve very well for all ordinary requirements, and can be put together at a very slight cost. Those who desire to make them for profit would have no difficulty in selling them at a remunerative price among botany and biology students.
Figs. 1 and 2 are section and plan respectively of the appliance, Fig. 3 showing the method of using it. The first thing needed is a circular disc of wood A, from 4 in. to 5 in. in diameter and 3/4 in. thick. A gas block, such as is used for fixing gas brackets to walls, suits the purpose admirably, and will be ready turned to shape, moulded on the edge, and polished. In the centre of this block bore a hole, 1 in. bare in diameter, entirely through the thickness. Next cut a circular piece B, of thin brass or zinc, to the same diameter as the top of the wooded block. Before screwing this plate to the block, drill a 5/8 in. hole in the centre of it, taking care to make it centre correctly with the hole in the block. Then file off the top surface of the plate, and smooth it with emery and oil.
New procure a blank cap C (Fig. 1)-that is, a piece of brass pipe with one closed end-of 1 in. external diameter, and file it off so as to make the length equal to the thickness of the wooden base. A hole is drilled through the closed end of the blank cap, tapped to suit the thread of a milled-head screw E, which should be about 3/16 in. in diameter. Cut the screw off, if necessary, to about 3/4 in. long, measured from under the milled head, and, by filing, or by turning it down in the lathe, form a step 1/32 in. deep and 1/4 in. from the end. This end of the screw is to pass through an inner cylinder D, as shown in Fig. 1, and after being inserted must be riveted over to prevent its withdrawal. The inside cylinder, for which another brass blank cap may be used, must be made to fit closely into the outside cap C, but with just enough play to allow it to work easily up and down. The inner cylinder D is cut so as to be 1/16 in. below the under side of the surface plate B - that is, 1/2 in. or 9/16 in. long (see Fig. 1).
When these various pieces have been fitted neatly together, the external cylinder C, with its fitings, is driven tightly into the 1 in. hole previously bored in the wooden base, until it comes up tight underneath the surface plate. The microtome is now ready for use. When the milled-head screw is turned, the inner cylinder will be raised or lowered within a limit of 1/16 in. the underneath side of the surface plate preventing it from raising farther.
Now a word as to the method of using the microtome. Suppose it is desired to make a very thin cross section of the stem of a plant. First melt together some white wax and olive oil, so as to form a solid block when cool. A small quantity can then be used as required. Take a little of this prepared wax, melt it, and, having turned the milled-head screw so as to lower the inner cup as far as it will go, pour the molten wax through the hole in the surface plate into the cup until the wax reaches the level of the surface plate; then, while the wax is still liquid, stand the plant-stem F (Fig. 3) upright in the middle, as shown. Allow the wax to cool, and then turn the milled-head screw about a quarter of a revolution, so as to raise the inner cup very slightly, and with it the wax and the embedded stem.
Now take an old razor, and grind one of its faces perfectly flat, and sharpen it up from one side only as if it were a chisel. Press the flat side of the razor in close contact with the metal plate, and push it forward, slicing off the top of the wax, and paring the stem level with the face of the plate. Then turn the milled-head screw very carefully for another quarter revolution, or less, so as to raise the inner cup and wax very slightly higher. Repeat the work with the razor, when it will be possible to pare off an extremely thin slice of wax, with a very thin section of stem embedded in the centre. Do not attempt to handle the delicate section with the fingers, but float it off the razor with a camel-hair brush into a little alcohol, which will dissolve the wax. The section may then be strained, and mounted on an object-glass in the usual way. It may be necessary to add that as the milled-head screw projects below the microtome, a hole should be bored in the bench top sufficiently deep to take the projecting milled-head screw, thus serving the double purpose of allowing the microtome to rest firmly on the bench, and preventing it from sliding along the bench when the razor is being used.-"Work," London.