MAKING SMEARS 51 



and nothing is better for this purpose than osmic acid. To use this material, 

 take a petri dish and place in it a couple of thin glass rods sufficiently far 

 apart to permit the slide to rest on them without the smear touching them. 

 Then place a drop or two of a solution of osmic acid, usually of 2 per cent 

 strength, in the bottom of the petri dish and replace the cover. It must be 

 emphasized that osmic acid fixes the mucous membrane of the nose and throat 

 just as readily as it does a smear, and every caution must be taken to avoid 

 inhaling the vapors of this material. As soon as the smear is made and before 

 it has time to dry, it is placed face down across the two glass rods, so that it is 

 exposed to the vapor but not to the liquid. The cover is then replaced on the 

 petri dish, and the slide left in place for 3 to 4 minutes, in the case of a thin 

 smear, or 5 to 10 minutes in the case of a thick one. Then it is transferred to 

 distilled water to await staining. 



It occasionally happens that one must fix a slide in one of the conventional 

 fluid fixatives. This is done with the same petri dish and glass rod setup as is 

 used for vapor fixation, but in this instance the fixative is carefully poured 

 into the petri dish, which must be level, until it has reached such a depth 

 that, when the slide is laid across the glass rods, the under side of the slide 

 with the smear on it is in contact with the fluid while the upper part is free 

 from fluid. If the smear is reasonably thin and is laid carefully in place, it 

 usually will not become detached. 



Staining Smears. Blood smears are stained so universally with one or 

 another of the methylene blue eosinate mixtures (see p. 27) that it comes as 

 something of a surprise to most people to learn that any stain which is suit- 

 able for sections may also be employed for smears. The advantage of these 

 mixtures for blood films is that the solvent methyl alcohol acts as a fixative, 

 so that the films, in effect, are stained and fixed in the same operation. Where 

 a blood smear is to be used for diagnostic purposes, these techniques are excel- 

 lent since the appearance of the various types of white corpuscles under this 

 treatment is known to every technician. For materials other than blood, there 

 is no limit to the type of staining which may be employed, although it must 

 be remembered that these very thin films require a stain of considerable in- 

 tensity if the finer structures are to be seen. 



