50 MATERIALS AND PROCESSES OF SLIDE-MAKING 



shown in Fig. l4, is placed on the drop. Capillary attraction, naturally, will 

 distribute the fluid along the edge of the second slide, which is then (Fig. 15) 

 pushed sharply forward until it reaches the end of the bottom slide. The ma- 

 terial of which the smear is made is thus dragged out behind the first slide 

 and distributed more or less uniformly on the under slide. A few people still 

 try to conduct the operation in the reverse manner by placing the second slide 

 on top of the first, sloping it at a reverse angle to that shown, and then endeav- 

 oring to push rather than drag the material across the lower slide. The objection 

 to this is that it results in crushing cells, although it must be admitted that it 

 frequently results in a more uniform distribution of the material. 



The method just described is the standard procedure for producing "thin 

 smears." These are necessary for those fluids, such as vertebrate blood or mam- 

 malian seminal fluid, which contain large numbers of objects requiring wide 

 separation for satisfactory study. 



There are a number of fluids, however, from which thick smears must be 

 made either because they contain relatively few cells, as in the case of inver- 

 tebrate blood, or because they contain parasites which are distributed relatively 

 sparsely through the material, as in the case of malarial diagnostic smears. 

 These thick smears are commonly made with the aid of a loop of wire held in 

 a needle holder of the type found in bacteriological laboratories. This loop is 

 dipped into the fluid to be examined. The material is spread with a rotary 

 motion in the center of the slide. This is very similar to the preparation of 

 smears of bacterial material which is described in some detail on page 100. 



Fixing Smears. Smears may be fixed by drying, by alcohol, or in one of 

 the conventional fixatives. When a smear is to be fixed only by drying, it is 

 dried by waving it in the air, as soon as it has been made, and then set aside 

 for subsequent treatment. This procedure is excellent in the case of objects, 

 such as bacteria or erythrocytes, which do not change their shape after drying; 

 or for materials, such as white blood corpuscles, which it is not desired to 

 preserve in their normal shape. No other object, however, can be considered 

 satisfactory unless it has been fixed. The simplest method of doing this is to 

 pass the smear, just as it is drying, through a jet of steam. This technique 

 has already been described (see p. 42) for mounting amebas and need not be 

 repeated here. 



All other smears should be fixed before they are dried, and it is something 

 of a problem to fix them without removing the material from the slide. It is 

 obvious that if the material is freshly smeared onto a glass slide and then 

 dropped into a fixative of some kind or another, it will be likely to be washed 

 off. The logical solution to the problem is to use a fixative in a vapor phase, 



