20            THE MICROTOMIST' S VADE-MECUM
begin to turn brown. There is no need to wash them out
before staining. Piero-carmine is a good stain to follow this
process, which is certainly, where practicable, the most elegant
that has hitherto been imagined.                           0.
When employed in aqueous solutions, osmic acid is used in      of 'a]
strengths varying from A   per cent. to 2 per cent. Solutions     6v,
of J per cent. to 1 per cent. have been very largely used, but
the tendency of modern practice seems to be towards weaker
solutions and longer immersion. For Infusoria 4 per cent.
for a few seconds; for Porifera -   to  1 per cent. for some
hours; for Mollusca 1 to 2 per cent. for twenty-four hours;        di
for epithelia T1 to 4- per cent. for an hour or two; for mero-
blastic ova , per cent. for twenty-four hours; for medul-
lated nerve-fibre Yiu to 1 per cent. for from twenty minutes to
two hours ; for tactile corpuscles 1 to 1 per cent. twenty-four   b neai
hours; for retina - to 2 per cent. for from   ten minutes to
twenty-four hours; for nuclei -flu to 2 per cent. for two or
three hours. Such figures as these will serve to give a general trance
idea of the practice, whilst more precise instructions will be
given when dealing with the tissues in detail.                     lt
Osmic acid stains all fatty structures perfectly and opaquely
black; it must therefore be avoided for tissues in which much
fat is present.
Osmic acid must be well washed out before proceeding to
any further steps in preparation; water may be used or gly-        a
cerin. Notwithstanding the greatest care in soaking, it fre- ma e
quently happens that some of the acid remains in the tissues,         of
and causes them to over-blacken in time. To obviate this it        fod
is necessary to wash them out in ammonia-carmine or picro-         a
carmine, or to soak them  for twenty-four hours in Miller's       of the
solution or in 0-5 per cent. solution of chromic acid, or in
Merkel's solution, or in a weak solution of ferrocyanide of
potassium or cyanide of potassium, or to bleach them (see
BLEACHING, No. 476, et seg.). All solutions of osmic acid must
be kept protected from the light even during the immersion of


K