814 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
as methyl-violet and blue, &c., are prepared. The concentration which 
appears to he most suitable varies between 1/100 and 1/500 according 
to the solubility and the colouring power of the substance. When 
insoluble in water the colour is dissolved in as small a quantity of 
alcohol as possible, and the solution is then diluted with water. 
The colouring solution is poured over the positive. After a few 
seconds the liquid penetrates the gelatin, which retains the colour. If 
the coloration is too intense, the proof is w r ashed with water. The 
decoloration is in this way generally effected slowly and regularly, 
and the washing is continued until the right tint is obtained. 
When the decoloration by water is not sufficient, alcohol is used. 
It is then much more rapid, so that the operation must be conducted 
with more care. The final washing is in all cases with ordinary 
water. 
To obtain a double coloration, as for example in the case of a 
microbe coloured red on a blue ground, the positive is first treated 
with a very intense colour. In the case of the microbe a 1 per cent, 
solution of magenta-red would be used. The proof is thus coloured 
in all its parts, the microbe deep red and the ground light red. A 
partial decoloration, first with water and afterwards if necessary with 
alcohol, is then effected. When the ground begins to lose its tint the 
proof is treated with the colour required for the ground. A w r eak 
solution, such as the aqueous 0 ■ 2 per cent, solution of cotton-blue, is 
used. For projection, it is necessary to varnish in order to get rid of 
the grained appearance of the surface. The projected images are then 
much more brilliant. 
C5) Microscopical Optics and Manipulation. 
Probable Limits to the Capacity of the Microscope.* — Dr. S. 
Czapski discusses the question of the limits to the resolving power of the 
Microscope. So long ago as the beginning of the century it was recog- 
nized that increased magnification was not the only thing necessary to 
render the details of a microscopic object clearly visible. With the same 
magnifying power, the same perfection in the correction for aberration, 
&c., and with the same method of illumination, systems having the 
larger angular aperture always showed superiority in definition and 
resolving power. The explanation of this “ specific function of the 
angular aperture ” came almost simultaneously from Abbe and Helmholtz. 
The theory of Helmholtz supposes the object to be self-luminous, so that 
it has not so direct a bearing as that of Abbe upon the ordinary Micro- 
scopic practice, in which the preparation is illuminated by reflected or 
transmitted light. However, the two theories, although thus divergent 
in their points of departure and in most of their consequences, lead in 
one point almost to the same result. With central illumination — i. e., 
according to Helmholtz, when the pencils of rays from the luminous 
points of the object occupy the whole aperture of the Microscope ; or, 
according to Abbe, when the object is met directly only by one small 
axial pencil — the resolving power according to both theories is deter- 
mined by the same formula. This formula shows upon what factors and 
* Zeitscbr. f. "Wiss Mikr., viii. (1891) pp. 115-55. 
