170 THE PREPARATION OF BLOOD FOR 



cover-glass thickness will be found, therefore, to play an important 

 part in blood examinations. 



The next step, continues the author, is the proper cleaning of 

 the covers. A small glass dish should be partly filled with battery 

 fluid (water, nine ozs. ; bichromate of potassium, one oz. ; sulphu- 

 ric acid, one oz.), and into this the covers should be dropped, one 

 by one, so that both sides of the cover may be wet by the fluid. 

 After remaining in this for twenty-four hours, the acid is poured off 

 and the covers are flushed en masse two or three times with water. 

 Then each should be taken separately and dropped into a dish of 

 distilled water, from which they are to be transferred, singly as 

 before, to alcohol (preferably pure methylic). A most convenient 

 receptacle for the alcohol and covers is a one-oz., square, screw- 

 capped bottle, in which they may be kept until needed for use. 



A very convenient instrument for drawing the blood is a small, 

 straight, surgical needle, several of which should be kept in a 

 phial of alcohol until needed. For two years or so he has used 

 needles made from an alloy of one part of iridium and two parts 

 of platinum. When required for use, the needle is sterilised at a 

 white heat immediately before and also after use. The blood may 

 very conveniently be taken from the tip of the finger, though some 

 writers insist that it is better to draw it from the lobe of the ear. 

 In either case the part should be thoroughly cleansed. 



Two forceps are required. One should be of the self-closing 

 variety, with flat, broad points, and with a spring sufficiently stiff" 

 to hold the cover firmly against moderate traction, The other may 

 be of any sort that will hold the cover nicely. 



A sufficient number — say six or eight — of the covers are 

 removed from the alcohol, thoroughly dried, and laid upon any 

 suitable support, projecting a little beyond it. One of the covers 

 is seized with the self-closing forceps and placed ready at hand. 

 The puncture is then made, and another cover is quickly taken 

 with the second forceps and applied to the droplet of blood as it 

 issues from the wound. The second cover is then laid on the 

 first, and the blood spreads out between them. A common fault 

 with beginners is taking up too much blood; but this will be cor- 

 rected after a little practice. As soon as the film is spread, the 

 projecting edges of the upper cover are taken between the thumb 



