Cites Cambridge Rocking Microtome
Description:

'The Cambridge Scientific Instrument Company have just introduced an improved and greatly simplified microtome for producing ribbons of section imbedded in paraffin (fig. 132).

The principle of the simplification is in the employment of a rotary instead of a sliding movement of the parts. The continuous moving silk band which is used in all previous forms is entirely done away with, and the ribbon of sections falls by its own weight direct from the razor on to a sheet of paper or on to the glass slide on which the sections are to be finally mounted.

The construction of the instrument is as follows:- Two uprights are cast on the base-plate, and are provided with slots at the top into which the razor is placed and clamped by two screws with milled heads. The inner face of the slot is so made as to give the razor that inclination which has in practice been found most advantageous. The razor is thus clamped between a flat surface and a screw acting in the middle of the blade, and the edge of the razor is consequently in no way injured.

The imbedded object is cemented with paraffin into a brass tube which fits tightly on to the end of a cast-iron lever. This tube can be made to slide backwards or forwards, so as to bring the imbedded object nearer to the razor ready for adjusting. The cast-iron lever is pivoted at about 3 in. from the end of the tube. To the other end of this lever is attached a cord by means of which the motion is given, and the object to be cut brought across the edge of the razor. The bearings of the pivot are V-shaped grooves, which themselves form part of another pivoted system.

Immediately under the first pair of V's is another pair of inverted V's, which rest on a rod fixed to two uprights cast on the base-plate. A horizontal arm projects at right angles to the plane of the two sets of V's; the whole being parts of the same casting. On the end of the horizontal arm is a boss with a hole in it, through which a screw passes freely. The bottom of the boss is turned out spherically, and into it fits a spherical nut working on the screw. The nut is prevented from turning by a pin passing loosely through a slot in the boss. The bottom of the screw rests on a pin fixed in the base-plate.

It will be seen that the effect of turning the screw is to raise or lower the end of the horizontal arm, and therefore to move backwards or forwards the upper pair of V's, and with them the leverand object to be cut. The top of the screw is provided with a milled head, which may be used to adjust the object to the cutting distance.

The distance between the centres of the two pivoted systems is 1 in. and the distance of the screw from the fixed rod is 6 1/4 in. The thread of the screw is 25 to the inch; thus, if the screw is turned once round the object to be cut will be moved forward 1/25 of 1/6 1/4 or 1/156 in.

The turning of the screw is effected automatically as follows:- A wheel with a milling on the edge is fixed to the bottom of the screw. An arm to which a pawl is attached rotates about the pin which supports the screw. This arm is moved backwards and forwards by hand or by a cord attached to any convenient motor. When the arm is moved forward the pawl engages in the milling and turns the wheel; when the arm is moved back the pawl slips over the milling without turning the wheel. A stop acting against the pawl itself prevents ant possiblity of the wheel turning, by its own momentum, more than the required amount. The arm is always moved backwards and forwards, between two stops, a definite amount, but the amount the wheel is turned is varied by an adjustible sector, which engages a pin fixed to the pawl and prevents the pawl from engaging the milling on the wheel. By adjusting the position of this sector the feed can be varied from nothing to about 5/32 of a turn, and hence, since the screw has 25 threads to the inch, the thickness of the sections cut can be varied from a minimum, depending on the perfection with which the razor is sharpened, to a maximum of 5/32 of 1/25 of 1/6 1/4 or 1/1000 of a turn.

'The practical minimum thickness obtainable with a good razor is approximately 1/40,000 in. The value of the teeth on the milled wheen are as follows:-

1 tooth of the milled wheel = 1/40,000 in. = .000625 mm.

2 teeth    "            "           = 1/20,000  "  = .001250  "

4   "        "            "           = 1/10,000  "  = .0025      "

16  "       "            "           = 1/2500     "  = .01         "

The movement of the lever which carries the imbedded object is effected by a string attached to one end of the lever. Thus string passes under a pulley and is fastened to the arm carryong the pawl. Attached to the other end of the lever is a string pulling downwards.

When the arm is moved forward the feed takes place, the string is pulled, the imbedded object is raised past the razor, and the spring is stretched. When the arm is allowed to move back the spring draws the imbedded object across the edge of the ra\zor, and the section is cut. The string is attached to the lever by a screw wehich allows the position of the imbedded object to be adjusted so that, at the endo fhte forward stroke, it is only just past the edge of the razor. This is an important adjustment, as it causes the razor to commence the cut when the object is travelling slowly and produces the most favourable conditions for the sections to adhere to each other.

The following are perhaps the most prominent advantages of this instrument. (1) The price is one-sixth of that of the original form. (2) Less skill is required from the operator, for the endless silk band is superseded, and the troublesome and difficult operation of lifting the first sections from the razor on to the silk band is entirely avoided; the ribbon of sections now falls of its own weight direct from the razor on to a piece of paper or glass slide placed to receive them, and by occasionally moving the paper forward any length of ribbon can be obtained. (3) The razor is fixed at what has in practice been found the most advantageous inclination and angle for cutting, and thus an unnecessary adjustment and waste of time is avoided. (4) The imbedded object is with great ease brought up to or away from the edge of the razor; first for large amounts by sliding backwards or forwards the brass tube on the cast-iron lever, then for smaller amounts by turning round the screw, when the pawl is out of gear, by means of a small milled head placed on top for this purpose. (5) There are no delicate working parts which can get out of order, and the whole instrument is easily taken apart for packing and is very portable.' (549-552)