^joumL^uTriS] Chloride of Gold in Microsco^py. 45 
YIII. — On the Use of the Chloride of Gold in Microscopy. 
By Thomas D wight, jun., M.D. 
Pekhaps no re-agent has of late years played so important a part 
in microscopy as chloride of gold. By means of it Conheim first 
demonstrated the terminations of the nerves in the cornea; and 
since it has been very generally used, particularly in investigations 
of the nerves. Its application is very difficult, and it is only after 
a long series of experiments and failures that proficiency is ob- 
tained. 
Having had considerable experience with this re-agent in the 
laboratory of Professor Strieker, in Vienna, and having obtained 
some very satisfactory results, I hope that a few words on its appli- 
cation may not be out of place. The chloride should be dissolved 
in distilled water, and the solution should never be stronger than 
the half of one per cent. The object to be examined should be as 
fresh as possible, and should remain in the fluid from three minutes 
to perhaps an hour, according to its affinity for the re-agent, during 
which time it assumes a pale straw colour. If the piece be small 
enough to be readily acted upon, ten or fifteen minutes is almost 
always sufficient. It is then laid in distilled water, to which just 
enough acetic acid has been added to give it the faintest possible 
re-action. In two or three days it wiU have become purple, verg- 
ing sometimes on blue, sometimes on red ; the latter is the least 
favourable. The preparation is now enclosed in glycerine, and 
improves for several days as the colour becomes deeper and as the 
finest fibres are the last to be affected. If the experiment has 
succeeded — for it sometimes unaccountably fails — the picture pre- 
sented is one of the most beautiful and instructive that can be 
imagined. The nerves, muscular fibres, and fibrous tissue appear 
black on the purple background. Epithehal ceUs are also coloured, 
but not so weU as by nitrate of silver. 
Although the colour makes fibres visible which are so fine that 
they can be seen by no other method, it does not determine their 
character. To prove beyond all doubt that a minute fibre is a 
nerve, we must be able to follow it to a larger branch. On a very 
successful preparation of the cornea of a frog, I observed nerve- 
fibres of such minuteness that with a magnifying power of nearly 
two thousand diameters it was impossible to follow them to their 
terminations. I particularly endeavoured to verify the connection, 
asserted by Kiihne but not generally accepted, between the nerves 
and the corneal corpuscles. With every advantage, such a connec- 
tion is very difficult to prove. I often thought I had found one ; 
but, when examined by a higher power, and placed in different 
