34 SCHULTZE, ON THE RETINA. 
In his researches on the retina M. Schultze has found the 
greatest advantage in the use of a solution containing 1 to +}>th 
of osmic acid (OsO,); and he recommends that a solution of 
that substance containing 1 per cent. should be kept at hand, 
which can be diluted at pleasure. Microscopic preparations 
made with it he prefers to keep simply in water. 
The black colour which is assumed by the preparation, even 
within a few minutes of its immersion, is at first uniform 
throughout. But subsequently the different parts of the 
retina exhibit slight differences, the optic nerve-fibres and 
the molecular and intergranular layers exhibiting the deepest 
tint. In frogs and fishes the deepest colour is seen in the 
outer segments of the “rods.” In this way may be obtained 
preparations in which the outer segment is of a deep black 
colour, whilst the inner is almost uncoloured, the line of 
demarcation between the two being very abruptly defined. . 
A similar difference is observable also in mammals, but not 
so constantly, and under circumstances which cannot at pre- 
sent be explained. But the demarcation between the segments 
is always well defined, and the author can recommend 
no better medium for the examination of the “rods” and 
“cones” than‘osmic acid. A special advantage of the osmic- 
acid solution is that it hardens the elements of the connective- 
tissue framework more slowly than the nervous; and another 
is that, except in very strong solutions, it does not produce 
granular coagulation either within or without the elementary 
parts of the retina. 
The observer is cautioned against the injurious effects of 
osmic acid upon himself, unless great care be taken. 
Another medium greatly employed by him is what he terms 
** Tod-serum,” or iodized serum, which is used for the immer- 
sion of fresh dissections of the eye and other parts—the most 
delicate tissues, such as the retina, remaining for a long time 
unaltered in it. It is prepared from the amniotic fluid of the 
calf, to which a sufficient quantity of tincture of iodine is 
added to give it a faint yellow colour. And he has found 
that an albuminous fluid of this kind may be kept unaltered 
for any length of time ifa very minute quantity of bromine be 
added to it. But as bromine acts very powerfully in causing 
cells, &c., to contract, the quantity added to the iodized 
serum must be less than will remove the whole of the yellow 
tint. 
[It is not improbable that a few drops of carbolic acid 
would answer the same purpose as the bromine, and perhaps 
the iodine also.] 
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