22 BLOOD CORPUSCLES. 



so ranch so, that they often appear to be surrounded by a 

 narrow zone of protoplasm ; (c) large masses of finely granular 

 protoplasm, which commonly are of irregular form, and in- 

 close bodies similar to the nuclei above described, varying in 

 number from five to twenty in each mass. 1 



Methods of Warming a Preparation. As in our fur- 

 ther study of the blood corpuscles it will be necessary to em- 

 ploy artificially increased temperatures, we proceed to describe 

 the methods employed for applying heat to preparations whilst 

 under microscopic observation. These methods are of two 

 kinds. The first is used when we wish to subject the prepa- 

 ration for an indefinite period to an increased temperature, to 

 which it has been graduallj r raised ; the second when we wish 

 to warm it suddenly, but for a very short period. To accom- 

 plish the first of these objects, a very simple contrivance, shown- 

 in Fig. 1, may be used. Take a cover-glass, and spread all 

 round the edge of its upper surface a thin layer of oil ; then 

 take another cover-glass of the same size as the first, place 

 on its centre the drop of the blood to be examined, and allow 

 it to fall on the glass previously prepared, edge to edge, with 

 the blood drop downwards. The drop will then occupy the 

 space between the two, inclosed by the layer of oil in such a 

 manner that it may be examined under high powers. The 

 preparation may then be readily lifted with the aid of a lancet- 

 shaped knife, and placed on the orifice of the copper plate (e). 

 The copper rod (g) is then gently warmed by means of a spirit- 

 lamp, a little cacao butter (or some other fat, the fusing point 

 of which nearly corresponds to the desired temperature) having 

 tieen previously placed on the copper plate, close to the prepa- 

 ration. As soon as the cacao butter begins to liquefy, the 

 flame of the lamp is diminished, or the lamp itself is removed 

 to a greater distance, until the heat communicated b}' it to the 

 plate through the rod is just sufficient to keep the fat from 

 solidifying. If it is desired to employ higher temperatures, 

 or to measure the temperature with greater exactitude, it is 

 necessary to have recourse to Strieker's warm stage. 



Strieker's Warm Stage. Of this there are two forms. In 

 one the mode of heating, and consequently of modifying the 

 amount of heat communicated, is that which has been already 

 described (see Fig. 2). From its simplicity it is well adapted 

 for the beginner, while it enables the more practised observer 

 to maintain any desired temperature within very inconsidera- 

 ble limits of variation. The other, in addition to the greater 

 exactitude which can be attained, has the advantage that, by 



1 Free nuclei of colored corpuscles, which may bo soon if tho prepa- 

 ration has been subjected to pressure, must not be confused with these 

 structures. 



