often happens in cutting tissues imbedded in paraffine, that the pressure of the knife causes the cylinder of the imbedding agent to twist round in the tube of the machine, and so cause considerable difficulty and annoyance. This evil is usually met by running a deep groove across the upper surface of the plug, and into this the paraffine sinks, and so is prevented from rotating. It will be found, moreover, that another difficulty of a kindred, though much more serious character, will frequently be encountered. During section the paraffine has a tendency not only to rotate, but also to become loosened from the subjacent plug, and to _rise_ in the tube of the microtome. When this happens the power to cut sections of uniform thickness has completely gone, for some will now be found to be many times thicker than others; in fact, the irregularity in this respect soon becomes so monstrous as to render it useless to prolong the sitting. In the ordinary run of microtomes no provision seems to have been made to meet this difficulty, and for this reason many instruments, of otherwise great merit, have their efficiency seriously impaired. Fortunately, this imperfection is easily remedied, all that is required being that the upper surface of the plug should be furnished with some kind of projection, having at its summit a table-like expansion, as shown at A in the figure. The imbedding paraffine, by penetrating beneath and around this, becomes firmly attached to the plug, and thus all risk of its rising is effectually avoided. If the student wishes to secure a really first-class instrument, none can be so confidently recommended as the freezing microtome of Professor Rutherford. In addition to its being the best instrument for carrying out the freezing method (§ 18), this machine is equally effective for cutting tissues imbedded in paraffine, or any of the other agents used for that purpose; indeed, whatever work a microtome _can_ do, _this one_ will perform. Footnote 4: Note D. Footnote 5: Note E. Footnote 6: Note F. [Illustration: SECTION OF MICROTOME-TUBE SHOWING ARRANGEMENT (A) TO PREVENT “RISE” OF PARAFFINE.] 9. _Æther Microtome._—A word here as to freezing microtomes, where the agent used is æther. Such as have fallen under our notice have not answered the expectations we were justly entitled to form of them. That it is possible to freeze a piece of tissue by their use is undeniable, but it is, as a rule, at an expenditure of such a quantity of æther (only the very best of which must be used) as to constitute it a very expensive proceeding. Another serious disadvantage they possess is, that if the supply of æther be intermitted for only a very short time, the already frozen tissue thaws with great and most inconvenient rapidity.