When sections of the softer substances are required, no instrument can be compared with “Valentin’s knife,” which consists of two steel blades lying parallel with one another and attached at the lower end. The distance of separation may be regulated at will by a small screw near the handle. When, therefore, a section is wanted, the substance must be cut through, and betwixt the blades a thin strip will be found, which may be made of any thickness, according to the distance of their separation. By loosening the screw the blades may be extended, and the section may be floated out in water if the damp will not injure it. The knife cuts much better if dipped in water immediately before use and also when the substance to be operated upon is wet, or even under water altogether; but care must be taken, after use, to clean the blades thoroughly and oil them before laying by, if the place is at all damp. This instrument is most useful in such subjects as anatomical preparations where the sections are required to show the position of the different vessels, &c.; but, as before stated, is very valuable for all soft substances. As an instance of this, it may be mentioned, that it is frequently used in cutting sections of sponges; but as these are often very full of spicula, &c., it is much better to press the sponge flat until dry, and then cut off thin shavings with a very sharp knife; these shavings will expand when placed in water. After this they may be laid betwixt two flat surfaces and dried, when they may be mounted as other dry objects, or, when desirable, in balsam. _Valentin’s knife_ is very much used in taking sections of skin, which are afterwards treated with potash solution, acids, &c., to bring out in the best way the different portions. Dr. Lister’s mode, however, of getting these is thus given in the _Microscopic Journal_:--“But I afterwards found that much better sections could be obtained from dried specimens. A portion of shaved scalp being placed between two thin slips of deal, a piece of string is tied round them so as to exercise a slight degree of compression; the preparation is now laid aside for twenty-four hours, when it is found to be dried to an almost horny condition. It then adheres firmly by its lower surface to one of the slips, and thus it can be held securely, while extremely thin and equable sections are cut with great facility in any plane that may be desired. These sections, when moistened with a drop of water and treated with acetic acid, are as well suited for the investigation of the muscular tissue as if they had not been dried.”