For counting the white blood corpuscles the same instrument is
generally used, but the blood is diluted 10 times instead of
100 times. It is advantageous to use a diluting fluid which
destroys the red blood corpuscles, but which brings out the
nuclei of the white corpuscles, so that the latter are more
easily recognised. For this purpose the solution recommended by
Thoma is the best--namely a half per cent. solution of acetic
acid, to which a trace of methyl violet has been added[1].

The results of these methods of enumeration are sufficiently exact, as
they have, according to the frequently confirmed observations of R.
Thoma and I. F. Lyon, only a small error. In a count of 200 cells it is
five per cent., of 1250 two per cent., of 5000 one, and of 20,000
one-half per cent.

There are certain factors in the practical application of these methods,
which in other directions influence the result unfavourably.

It has been found by Cohnstein and Zuntz and others that the blood in
the large vessels has a constant composition, but that in the small
vessels and capillaries the formed elements may vary considerably in
number, though the blood is in other respects normal. Thus, for example,
in a one-sided paralytic, the capillary blood is different on the two
sides; and congestion, cold, and so forth raise the number of red blood
corpuscles. Hence, for purposes of enumeration, the rule is to take
blood only from those parts of the body which are free from accidental
variation; to avoid all influences such as energetic rubbing or
scrubbing, etc., which alter the circulation in the capillaries; to
undertake the examination at such times when the number of red blood
corpuscles is not influenced by the taking of food or medicine.

It is usual to take the blood from the tip of the finger, and only in
exceptional cases, _e.g._ in oedema of the finger, are other places
chosen, such as the lobule of the ear, or (in the case of children) the
big toe. For the puncture pointed needles or specially constructed
instruments, open or shielded lancets, are unnecessary: we recommend a
fine steel pen, of which one nib has been broken off. It is easily
disinfected by heating to redness, and produces not a puncture but what
is more useful, a cut, from which blood freely flows without any great
pressure.

The literature dealing with the numbers of the red corpuscles in health,
is so large as to be quite u