OF MICROSCOPIC OBJECTS. 103 



will by a small screw near the handle. When, therefore, a sec- 

 tion 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 riot 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 com- 

 pression ; 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 investiga- 

 tion of the muscular tissue as if they had not been dried." 



There are many who almost confine their attention to polarized 



