1884.] Microscopy and Histology. 957 
Both the brain and spinal cord were entirely separated from 
the body, and, with their membranes, placed in iodine-tinted al- 
cohol until they had acquired a slight degree of consistency—from 
six to twelve hours, They were then transferred to a 3: 100 so- 
lution of bichromate of potash, with a small piece of camphor, in a 
tightly-corked wide-mouthed bottle, and allowed to remain until 
ready for cutting, renewing the solution every two weeks. 
The time required for the hardening process varies considerably 
in different animals, and this variation is more dependent upon 
the class of animal, than upon the relative dimensions of the 
specimens, 
For example, on the same day I placed the brain of a large 
rattlesnake with that of a small salamander in the same bottle, 
andat the end of six weeks the former was ready for section, 
whilst the latter was not sufficiently hard until a month afterwards. 
By thus employing the same reagent in all cases, I have been 
le to note constant differences in the action of both the hard- 
ening and the coloring agent, carmine. : 
Perhaps the most striking illustration of this is furnished by the 
nervous centers of tailed batrachians, which, while they stain very 
readily, invariably require about a third more time to harden than 
specimens from the other orders. Specimens from ophidians stain 
less satisfactorily than those from any other of the classes which 
I have studied, while with the spinal cords of alligators, turtles, 
aai frogs, failure to obtain good results in this particular is very 
In all cases the sections have been stained after cutting, injury 
from excessive handling being wholly avoided by the use of siphon 
tubes to remove the alcohol and washings. For producing trans- 
parency, oil of cloves has been used, and the mounting has been 
done under thin, clear covers, in a solution of Canada balsam in 
chloroform, 
All the negatives have been made on glass thoroughly. cleaned 
and lightly coated with a solution of wax and benzole, so that the 
collodion film, previously made adherent to thin sheets of gela- 
one could be safely removed from the plate. The flexible nega- 
us obtained are well adapted to the artotype process, and, 
as they can be indefinitely preserved between the leaves of an 
oy scrap-book, are very desirable for a series of illustrations. 
gone, the original negatives on glass, the ‘wet collodion 
aiie dr the sulphate of iron developer, has been exclusively 
oyed. | 
li he prints correspond exactly with the negatives, both in out- 
ne and detail. No distinction occurs as in silver printing in 
erst process the paper is subjected to prolonged washing. 
._ 1. Many of the photographs the gray substance appears lighter 
in shade than the white substance. This appearance 15 due to a 
aes degree of transparency of the gray substance in these sec- 
