When a specimen has a very irregular outline, it cannot be very successfully embedded in carrot; paraffin will then be found to be more suitable. Place the tissue in the well of the microtome in the proper position, pour in enough melted paraffin to cover it, and put it by to get cold and hard before attempting to cut sections.
[Illustration: Fig. 234.--The Cambridge Rocking Microtome.]
_Cambridge Rocking Microtome._--This new pattern Cambridge Rocking Microtome (Fig. 234) possesses advantages over other instruments in use for cutting flat sections, and not parts of a cylindrical surface. The tube containing the paraffin is 30 millimetres in internal diameter instead of 20 millimetres, as in the earlier forms. The forward movement is also increased, so that an object 12 millimetres long can be cut throughout its whole length. It is provided with a dividing arc for reading off the thickness of the section in thousandths of a millimetre. The razor may be fixed either with its edge at right angles to the direction of motion of the object, or diagonally, for giving a slicing cut. The object can also be raised and fixed in position clear of the razor.
This microtome has both steadiness and stiffness in its geometrical arrangement and bearings, while the simplicity and efficiency of its mechanism for advancing the section between each stroke of the razor is remarkable. Although it may appear more complicated at first sight, it is found not to be so when brought into use.
[Illustration: Fig. 235.--Cathcart’s Microtome.]
[Illustration: Fig. 235_a_.--Section Cutting Holder for Microtome.]
_Cathcart’s Freezing Microtome._--This is a convenient and useful microtome for freezing purposes. Since its first introduction it has been much improved. The clamping arrangements give steadiness, and the principal screw is more effective; the freezing-plate is circular, and the arrangements made for preventing the ether from reaching the upper plate secures the object in view. This instrument can now be used for embedding as well as freezing. The directions for freezing are as follows:--
1. Place a few drops of mucilage (one part gum to three parts water) on the zinc plate.
2. Take a piece of the tissue to be cut, of about a quarter of an inch in thickness, and press it into the gum.