240 CLINICAL BACTERIOLOGY AND H/EMATOLOGY 



This is perhaps the most difficult part of the process, and the 

 exact amount which must be allowed to fall on to the slide 

 can only be learnt by experience. It is better to take too little 

 rather than too much. You need not cover the disc completely, 

 as long as the rulings and a fair margin round them are 

 covered. 



Cover the slide in this way : Place your finger at the side 

 on the glass square on the slide, and apply the cover-glass, 

 letting it rest against your finger; lower it gently in place 

 with a needle or other suitable object. When it is in place 

 press it gently with the needle at each corner in succession, 

 and look at it obliquely, so as to see the light reflected from 

 the surface. If the slide and cover-glass are in sufficiently 

 close contact, you will see Newton's rings (looking like the 

 eye of a peacock's feather) round the point at which you are 

 applying pressure. If you do not see this, the inference is 

 that there is some dust between the slide and cover-glass; 

 you must clean both and begin again. 



It is a great advantage to clip the cover-glass to the 

 counting-chamber until the corpuscles have had time to 

 settle. When this has taken place the depth of the chamber 

 is immaterial, and it does not matter if the cover-glass rises 

 somewhat. The simplest 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. 47). 



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. 46, r). If this happens, 

 or if there are any bubbles under the cover-glass, you must 

 begin again. 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 

 beginners, and merits a short description. Place the slide 

 under the microscope, taking care to get it accurately cen- 

 tred, and examine it with the lower power. You will find 

 that the central disc is ruled into squares like a chess- 



