SIMPLE WARM STAGE. 31 



thoroughly with a needle about an equal amount of 

 blood obtained from the finger, as in Prep. 1, and 

 carefully cover the mixed fluid with another cover- 

 glass, somewhat smaller than the first. If there is 

 now not enough fluid to fill the space between the 

 two glasses, add a little more salt solution at one 

 edge of the smaller cover-glass; but if, on the other 

 hand, there is too much, soak up the excess with a 

 small piece of blotting-paper. A very small camel- 

 hair pencil which has been dipped in olive-oil is now 

 to be drawn gently along each edge of the smaller 

 glass; this will prevent evaporation from the ed^es, 

 which would otherwise quickly ensue on warming 

 the preparation. The dilution of the blood with 

 salt solution prevents in great measure the aggrega- 

 tion of the red corpuscles, while at the same time in 

 no way interfering with the movements of the white 

 ones; moreover, it is favorable to the changes which 

 the above-mentioned masses or colonies of discoid 

 particles undergo, if any such happen to be present. 

 The glass slide which bears the copper plate having 

 been clamped on to the microscope stage, the prepa- 

 ration thus made is placed upon the copper, and, 

 having been brought in focus, one or more white 

 corpuscles are selected for observation a high mag- 

 nifying power being used. The rod is now heated 

 near its end by a small spirit-lamp, and the heat is 

 conducted by the rod to the copper plate, and from 

 this is transmitted to the preparation, close to which 

 a small fragment of a mixture of white wax and 

 cacao-butter, previously made, and melting at about 

 30 C., is to be placed upon the copper (Fig. 7). The 

 lamp is now gradually approached along the rod 

 until it arrives at a spot the heat transmitted from 

 which is just sufficient partially to melt the frag- 

 ment, and it is then left burning at that spot, for 

 since the fat employed melts at about the tempera- 

 ture of the body, we know that the preparation will 

 now be also warmed nearly to the same point. 



It will be seen that as the preparation begins to 



