xxvi INTRODUCTION. 



indispensable to the injector. They should be short, and not heavy; otherwise the vessel 

 may be torn by their weight. 



Surgical " dressing-forceps " are also frequently of use; and long tf oesophagus-forceps " 

 with scissor-handles are serviceable for removing portions of plants c. from large jars or 

 glass vessels. 



Needles. For separating the parts of minute objects, fine points are requisite ; these are 

 found in common needles of moderate size fixed by one end into the handle of a water- 

 colour brush. These are easily prepared : the needle is cut in half by cutting-pliers ; the 

 blunt end is then forced into the stick, about half an inch in length being left projecting. 

 Surgeons' " cataract-needles " ground down are elegant instruments of this kind, but they 

 require to be shortened. For the minute dissection of objects, the mounted needles require 

 pointing on a hone. 



A stout sable-hair or fine bristle, inserted into a slender wooden handle, is frequently of 

 use in isolating minute bodies, as Diatomacefle, which would be broken by any other instru- 

 ment. It is used thus : suppose we have a number of Nawcula, or the like, in a bottle, 

 mixed with other bodies, and we wish to isolate one for preservation. A small quantity 

 of the deposit is taken up with a dipping-tube, and allowed to escape upon a slide in such 

 manner as to form a narrow stripe upon it. This is then examined with the lowest power 

 with which the object can be distinguished, and one near the margin of the liquid stripe is 

 selected, and may easily be removed with the mounted bristle (under the microscope) 

 beyond the margin of the liquid. The remainder of the liquid is then wiped away with a 

 cloth, a little distilled water added to the small quantity of liquid left containing the object, 

 and the latter moved with the bristle into the middle of the slide. The liquid is then driven 

 off by heat, and the object is left on the slide ready for mounting. Or, when the matter is 

 dried upon the slide, any one of the minute objects being lightly touched with the dry 

 bristle will adhere to it ; and by gently pressing or rotating the bristle upon the middle of 

 a new slide, the object will readily be transferred to the latter. The Diatomacea) may be 

 easily isolated in this way. 



Knives. Ordinary dissecting-kuives or scalpels. The handles should be sufficiently 

 large to allow of being firmly held. 



A particular and most useful kind of knife for producing thin sections of soft bodies is 

 that known as " Valentin's knife." It consists of two or sometimes three blades with their 

 fiat surfaces parallel, set in a handle. The blades can be fixed at any distance apart, 

 according to the thickness of the section required. It is drawn across and through the 

 substance, from heel to point ; the section remains between the blades, and is then removed, 

 either with forceps, or the blades of the knife are opened under water, and the section 

 floated upon a slide immersed in the liquid. In the latter case, the action of the water 

 upon the tissue must not be overlooked. Valentin's knife is absolutely indispensable in 

 the examination of animal bodies. Some sections, especially of plants, are best made with 

 a razor. Many sections can only be made by the aid of the " section-cutter " or microtome, 

 which is described under PREPARATION. 



Black and white Disk. A disk 3 or 4 inches in diameter, made of seasoned wood, and 

 upon one face of which a piece of white paper or card-board has been fastened by paste or 

 glue. One half of the paper or card-board is coloured black ;t he other is left white. This 

 is very useful in dissecting or separating minute portions of tissues ; if these are white, 

 they become much more easily distinguished than usual when placed (on a slide) over the 

 black part of the disk ; if they are dark, over the white portion. 



