32 Mr Ponder, On the examination of 



of the slide and coverslip, which form the glass floor and roof of 

 the blood-chamber ; it is now only necessary to clean away the 

 clot. To do this, the coverslip is removed while under the surface 

 of the warm saline solution by passing beneath it the point of a 

 knife or needle, and what remains of the clot and plasticine is 

 scraped away from the slide with a small knife. The slide should 

 now be well washed in the warm saline until all free red cells 

 have been rinsed off; this is shown to have taken place completely 

 by the disappearance of any reddish colour, and on holding up the 

 slide to the light a grey film can be seen, which consists entirely 

 of leucocytes, adhering to the surface of the slide. 



If now only a temporary preparation is needed, it is merely 

 necessary to take the slide out of the warm saline bath, super- 

 impose a coverslip, taking care that plenty of saline lies beneath 

 it, and examine it on the warm stage under the microscope. If, 

 however, the preparation is to be kept some time, or it is desired 

 to bring in contact with the leucocytes some other fluid such as 

 serum, containing a suspension of bacteria, it is necessary to 

 construct on a coverslip, a plasticine chamber, similar to the one 

 originally described for obtaining the leucocytes — though in this 

 case with walls made as thin as possible — and having carefully 

 filled this with a large drop of the saline, or fluid, with which it is 

 desired to bring the leucocytes in contact, to press the slide down 

 on it, so that the chamber is completely filled with the fluid, and 

 the leucocytes on the surface of the glass become situated on its 

 floor. 



As an alternative preparation, the coverslip of the original 

 blood-chamber which is also covered with leucocytes, may be 

 taken, cleaned as described above, and pressed down similarly on 

 a chamber (made this time on a slide) which contains a drop of 

 saline or the fluid with which it is desired that the leucocytes 

 shall come in contact ; the leucocytes in this case are on the 

 under surface of the coverslip, so that the preparation resembles 

 that of a " hanging drop." 



The actions of the leucocytes can by this method be studied in 

 two ways. Firstly, they may be allowed to live and move in the 

 primary blood-chamber for some hours, the preparation being kept 

 in an incubator (evaporation does not take place, as the blood dries 

 across the narrow entrance and seals it), and then, when it has 

 been cleaned, fixed, and stained, the positions the leucocytes have 

 taken up, with regard to any foreign substance, can be observed. 

 Secondly, if the primary preparation is only incubated for ten 

 minutes, and the leucocytes are then transferred to a second 

 chamber, we have a means whereby their movements and inter- 

 actions with foreign substances can take place while under 

 observation. 



