preparation
may last as long as required, and can be repeated at any time. On the
contrary, the examination of the wet preparation is only possible at the
bedside, and must be conducted within so short a time, on account of the
changeability of the blood, clotting, destruction of white corpuscles
and so forth, that a searching investigation cannot be undertaken. In
addition the preparation and staining of dried blood specimens is
amongst the simplest and most convenient of the methods of clinical
histology. In the interest of its wider dissemination, it will be
justifiable to describe it more in detail.

We must also mention here the use of the dry preparation in the
estimation of the important relation between the number of the red and
of the white corpuscles; and also of the relative numbers of different
kinds of white blood corpuscles.

For this purpose, faultless specimens, specially regularly spread, are
indispensable. Quadratic ocular diaphragms (Ehrlich-Zeiss) are
requisite, which form a series, so that the sides of the squares are as
1:2:3 ... :10, the fields therefore as 1:4:9 ... :100. The eye-piece
made by Leitz after Ehrlich's directions is more convenient, in which,
by a handy device, definite square fractions of the field can be
obtained. The enumeration is made as follows. The white blood corpuscles
are first counted in any desired field with the diaphragm no. 10, that
is with the area of 100. Without changing the field, the diaphragm 1,
which only leaves free a hundredth part of this area, is now put in, and
the red corpuscles are counted. The field is then changed at random, and
the red corpuscles counted in a portion of the area which represents the
hundredth of that of the white. About 100 such counts are to be made in
a specimen. The average of the red corpuscles is then multiplied by 100
and so placed in proportion with the sum of the white. If the white
corpuscles are very numerous, so that counting each one in a large field
is inconvenient, smaller sections of the eye-piece 81, 64, 49, etc. may
be taken.

The important estimation of the percentage relation of the various forms
of leucocytes is effected by the simple "typing" of several hundred
cells, a count which for the practised observer is completed in less
than a quarter of an hour.

[alpha] Preparation of the dry specimen.

To obtain unexceptionable preparations cover-slips of particular quality
are necessary. They should not be thic