18 BLOOD CORPUSCLES. 



2dly, collect the blood on the cover-glass by bringing it into 

 contact with the drop, then place it on the slide with its clean 

 surface upwards. By following either of these methods, 

 the introduction of air-bubbles will be avoided, which would 

 otherwise be a source of difficulty to the beginner. The drop 

 should be neither too small nor too large. The following in- 

 conveniences arise from its being too large: 1. The thin cover- 

 glass does not lie steadily in its place, but floats on the drop 

 in such a way that, by the slightest movement of the table, 

 currents are produced in the liquid which render observation 

 difficult or impossible. 2. If it is desired to keep the preparation 

 for a length of time under observation, it is necessary to adopt 

 some means to prevent the liquid from evaporating ; for, if this 

 is not done, it gradually dries from the edges, and soon be- 

 comes unfit for the observation of amoeboid movements, we 

 therefore inclose the preparation in oil, as will be immediately 

 described, and experience shows that, by so doing, the move- 

 ments may be watched for twelve hours or more continuously 

 a time which is sufficient for a complete study of the phe- 

 nomena in question. This, however, cannot be done unless 

 the drop is very small. 3. If high powers are used, the front 

 glass of the objective comes into contact with the cover-glass, 

 and produces currents due to pressure. 



If, on the other hand, the drop is too small, the elements 

 are pressed upon by the cover-glass, and thereby subjected to 

 unnatural conditions. No definite rule can be given as re- 

 gards the size of the drop, which must of course vary with 

 that of the cover-glass. 



The mode of surrounding a preparation of blood with oil is 

 as follows : Take a drop with a glass roVl or camel-hair pencil, 

 and let it fall at the very edge of the cover-glass in such a 

 way that, although most of it is on the surface of the slide, it 

 covers a little of the cover-glass also. Then incline the glass 

 slide slightly, and with the rod lead the oil from the drop 

 along the edge of the cover-glass, taking care not to press 

 upon the latter. If one drop of oil is insufficient, of course 

 another must be added. Take great care to avoid sn/earing 

 the cover glass too far ; for by so doing, the space available 

 for observation may become inconveniently limited. 



Having thus obtained a preparation of blood entirely pro- 

 tected from evaporation, we are ready to begin our study of 

 the colorless corpuscles. 



Varieties of Colorless Corpuscles. As soon as we 

 have brought a field contain ing blood into focus, we see, in 

 addition to a multitude of colored blood disks, to which we at 

 present pay no attention, a greater or less number of color- 

 less corpuscles, which themselves differ from one another both 

 in size and aspect, and in their property of spontaneous 



