INTRODUCTION. 3 



the parts can be separated by teasing, are often better than 

 any cutting instruments. They are indispensable in the ex- 

 amination of very young flower-buds, and of great use in the 

 isolation of tissues under the dissecting microscope. 



8. Sufficiently thin sections of soft parts may be made by any 

 keen-edged knife. A razor of good quality is generally to be 

 preferred to the ordinary dissecting scalpel, since its wide and 

 stiff blade can be held with greater steadiness, and its steel 

 admits of as sharp an edge. As a rule, the razor should be 

 dipped in water before using, as this permits the steel to pass 

 more easily through tissues. 1 If the parts from which sections 

 are to be made are too small to be held in the fingers, they can 

 be firmly seized between slices of pith. It is often convenient 

 to imbed the object in paraffin or in an alcoholic solution of 

 soap. 2 These melt below the temperature of boiling water, but 

 are solid at ordinary temperatures, and the latter, if properly 

 made, is transparent. A little of the melted imbedding sub- 

 stance is poured into a small cone of glazed paper, and when it 

 begins to cool, the object is placed in the middle of the mass. 

 Upon complete cooling it is firmly held therein. 



Before putting the object into paraffin it should first be satu- 

 rated with alcohol, and this replaced by benzol or oil of cloves, 

 in order to enable the paraffin to hold the specimen firmly. The 

 paraffin may be dissolved away from the sections b} f application 

 of benzol, oil of cloves, or turpentine (see also 110). 



9. Thin sections are best removed from the knife by a 

 camel's-hair pencil, and are to be placed at once in water or 

 some other liquid. Except in certain cases, water ma}' be used 

 as a medium for the preliminary examination of sections. 



10. Microtome. Any of the simpler microtomes, or section- 

 cutters, will be convenient in much histological work, and of 

 great use in the preparation of a series of sections from any 

 very minute object, since this permits them all to be of exactly 

 the same thickness. 



11. Measurements. Microscopic objects are measured by 

 micrometers. The eye-piece micrometer can be more rapidly 

 used than one on the stage of the instrument; and if its value 



1 Advantage is frequently gained by moistening the edge of the knife with 

 dilute potassic hydrate before dipping it in water, thus removing traces of 

 oil which may have adhered to it during sharpening. But potassic hydrate 

 should not be used in this way if reagents are to be subsequently employed. 



2 Made by dissolving enough of any good transparent soap in hot alcohol, 

 to form, upon cooling, a firm, clear mass. 



