ESTIMATION OF THE RED CORPUSCLES 205 



method is to use four Cornet's forceps (or even only two, applied- 

 at opposite sides), as in the figure. Where this is done an 

 ordinary No. i cover-glass may be used instead of the special 

 thick one. Newton's rings should appear round the tips of the 

 forceps, as shown in the illustration. The preparation should 

 stand for five minutes to allow the corpuscles to settle, when the 

 forceps are removed and the count made (see Fig. 42). 



If you have taken the right amount of fluid, the drop should 

 extend exactly to the edge of the central glass disc, but should 

 not run over into the " moat " (Fig. 41, r). If this happens, or if 

 there are any bubbles under the cover-glass, you must begin again. 



Fig. 42. — Counting Chamber with Cover-glass clipped into Position. 



If the drop does not quite extend to the edge of the central disc, 

 no great harm is done. 



4. Focussing the Specimen. — This is somewhat difficult for be- 

 ginners, and merits a short description. Place the slide under the 

 microscope, taking care to get it accurately centred, and examine 

 it with the low power. You will find that the central disc is ruled 

 into squares like a chess-board (Fig. 41, c). Get these squares 

 into the centre of the field (see Fig. 43). 



Do not forget you are dealing with an unstained object ; use a 

 flat mirror and a small diaphragm. The examination is often 

 easier if artificial light is used. 



Now turn on the high power (|^-inch or -f-inch), and screw it 



