r, differ considerably one from
another. On the one hand forms poor in haemoglobin, on the other very
small forms occur, which by the wet method of counting cannot even be
seen.

Apart even from these extreme forms, 1,000 =red blood corpuscles of
anaemic blood are not physiologically equivalent to the same number of
normal blood corpuscles=. Hence the necessity of closely correlating the
result of the count of red blood corpuscles with the haemoglobinometric
and histological values. The first figure only, given apart from the
latter, is often misleading, especially in pathological cases.

It is therefore occasionally desirable to supplement the data of the
count by THE ESTIMATION OF THE SIZE OF THE RED BLOOD CORPUSCLES
INDIVIDUALLY. This is effected by direct measurement with the ocular
micrometer; and can be performed on wet (see below), as well as on dry
preparations, though the latter in general are to be preferred on
account of their far greater convenience.

Nevertheless the carrying out of this method requires particular care.
One can easily see that in normal blood the red corpuscles appear
smaller in the thicker than they do in the thinner layers of the dry
preparation. We may explain this difference as follows. In the thick
layers the red discs float in plasma before drying, whilst in the
thinner parts they are fastened to the glass by a capillary layer.
Desiccation occurs here nearly instantaneously, and starts from the
periphery of the disc; so that an alteration in the shape or size is
impossible. On the contrary the process of drying in the thicker
portions proceeds more slowly, and is therefore accompanied by a
shrinking of the discs.

Even in healthy persons small differences in the individual discs are
shewn by this method. The physiological average of the diameter of the
greater surface is, according to Laache, Hayem, Schumann and others, 8.5
mu for men and women (max. 9.0 mu. min. 6.5 mu.) In anaemic blood the
differences between the individual elements become greater, so that to
obtain the average value, the maxima, minima, and mean of a large number
of cells, chosen at random, are ascertained. =But with a high degree of
inequality of the discs this microscopical measurement loses all
scientific value.=

However valuable the knowledge of the absolute number may be for a
judgment on the course of the illness, it gives us no information about
the AMOUNT OF HAEMOGLOBIN IN THE BLOOD, which is the