NOTES AND MEMORANDA. 
337 
can afterwarils, by shaking small pieces of tlie grey substance in a 
little water in a reagent-glass, obtain very superior isolated ganglion- 
cells. When you think that it has been shaken enough, put a little 
glycerine to the water, and some drops of the concentrated solution of 
the piero-carminate of soda. Then let the glass remain undisturbed 
for a day or two. After that time the ganglion-cells, by reason of 
their greater speeific gravity, will have sunk either to the bottom of 
the glass or will be found in the lower strata of the water. What 
swims on the top is generally only shreds of connective tissue, in which 
there may be a ganglion-cell here and there. Generally the ujoper 
j^ortion of the fluid may now be poured off without much being lost. 
If much fluid is left, repeat the process after some time. The small 
remaining quantity of diluted red-coloured glycerine with the cells 
and shreds of connective tissue is then shaken in a small watch-glass, 
a few drops of glycerine added, and then placed in a sulj^huric acid 
drying apparatus ; at the end of two days the whole of the water has 
generally evaporated, and the shreds of tissues and cells lie (deeply 
stained) in glycerine pretty free from water, in which they may be 
kept as long as is wished. Gradually the stain seems to be attracted 
more and more to the tissue elements, and the fluid becomes almost 
colourless. In the watch-glass may now be seen under the microscojje 
the finest ganglion-cells stained deep red, and thereby easily to be 
discovered amongst the shreds of connective tissue, which are only 
weakly stained. The ganglion-cells cannot be Ashed out with a 
needle under a dissecting microscope and transferred to a mounting 
glass for mounting, as the numerous continuations almost always fix 
them. It is far better to place a drop of the fluid, in which there will 
generally be some cells, on the mounting glass, and then detach by 
means of a needle under the dissecting microscope the useless shreds 
of connective tissue, if possible without touching the cells. It is 
surprising how much easier the ganglion-cells are found after the 
staining, and how many more are seen. The large cells may be 
recognized with the naked eye. As any transfer of the ganglion-cells, 
however carefully made, always does some damage, the plano-concave 
slips, such as frequently accompanied the older microscopes, are best 
suited for the purpose of observation. In this case, however, only the 
weaker systems can be employed. As the cells will keep for any time 
in the glycerine, this method is well suited to provide a stock for a 
course. 
In other respects, picro-carminate of soda is not suited, according 
to my experience, to replace picro-carmine, as it possesses too weak a 
power of staining.* 
“ Ultimate Limit of VisionU — Dr. Fripp, of Bristol, the well- 
known translator of the papers of Professor Abbe and Helmholtz on 
the theory of microscopic vision, and the author of several valuable 
papers on that subject in the ‘ Proceedings of the Bristol Naturalists’ 
Society,’ writes as follows in regard to a suggestion made to him that 
Mr. Ballinger’s measurement of the flagellum of Bacterium termo, the 
* Dr. Schiefterdecker, in ‘ Arcliiv f. Mik. Aiiat.,’ vol. xv. p. 38. 
