File:EB1911 Microtomy - Large Sliding Microtome.jpg

English: In 1910 the Cambridge Scientific Instrument Company issued a new microtome designed primarily for cutting larger sections than was possible in their earlier forms, which respectively dealt with sections 12×20 mm. and 30 mm. in diameter; the new instrument cuts sections measuring 150×120 mm. (6×4⅜ in.) embedded in paraffin or celloidin and of a thickness varying from 0.002 to 0.06 mm., each division of the scale being equal to 0.002 mm. and the total distance of automatic feed being 21 mm. The construction and action of the instrument can be understood by referring to the figure; a detailed description is given, since the same principles are utilized to a greater or less extent in all sliding microtomes. The object to be cut, having been embedded in a suitable preparation A, is fixed to a wooden block which is attached by clamps to the object-holder B. The object-holder is provided with mechanism by means of which the height of the block is determined; this is effected by mounting the holder in a cup-shaped socket at the extremity of a brass pillar E, which can be raised or lowered and fixed in any position by a clamp. In addition, the direction in which a section is cut can be varied by adjusting the four screws, one of which is shown at C, which orientate the block. The object holder and feeding mechanism are carried on a sliding carriage which rests at three points on two guides in the frame N, N1 of the instrument; and in order to secure easy running the necessary lubrication of the bearing surfaces is provided for by a groove in which oil is placed. The motion of the carriage in either direction is effected by the handle G, connected to a system of levers H, which, being constructed on geometrical principles, prevent any side-play and ensure a uniform motion. The arrangement for determining the thickness of the section cut consists of a stop-pin, which, operating through the ratchet M, causes a toothed wheel to revolve, which in turn raises the pillar K; the amount of the motion can be read off by an index. On the return stroke of the sliding carriage the stop-pin is again actuated in such a manner that just before the knife R reaches the object-holder the mechanism depresses this part of the instrument so that the knife is not fouled; and after its passage the object-holder is raised to the position appropriate for taking the next section. The knife R is rigidly set in two heavy brass clamps adjustable by the screws S, and these clamps are attached to the frame of the instrument by the screws T. The angle which the cutting edge makes with the frame is also adjustable, and by means of a small angular scale engraved on the knife-holders any setting can be easily determined or repeated. The knife is flat on one side and hollow-ground on the other. In using the microtome it is essential that the cutting edge of the knife points towards the end of the instrument where the handle is placed; the hollow-ground face should be uppermost, and the flat surface should not be exactly horizontal but slightly inclined so that the lower facet of the cutting edge is parallel to the frame. As to the relation of the position of the knife to the direction of motion, it is the usual practice, when paraffin sections are to be taken, to have the cutting edge at right angles to the motion; when, on the other hand, celloidin preparations are being cut, the knife must be set obliquely across the frame, an angle of 30° being convenient. This oblique setting is also recommended for paraffin sections. In addition it must be remembered that celloidin preparations always require lubricating when being cut, and it is also necessary to keep both the knife and the preparation constantly moistened with either 80% alcohol or with cedar-wood oil.