LIPPMANN HELIOCHROMES—CAJAL. ‘Q41 
The grain is spherical, of homogeneous appearance, and of a color 
depending upon the duration of exposure, the hygrometric state of 
the atmosphere, and the developer. Generally, the grain of the less 
exposed parts is bluish gray; at the correctly exposed places it is of 
a light chestnut-brown color; when overexposed, fine greenish yellow 
or pale ocher-colored grains occur. Strongly solarized parts are 
always a clear bright yellow. It may be as well to point out that these 
colors are not dependent on the wave length of the hght, but on the 
duration of the action of the latter; and they only appear on the in- 
tensified plates; the action of the mercury chloride is not only’ to 
enlarge the grain, but to give it a uniform character and opacity and 
a more or less gray tone. The color of the grain alters also in rainy 
weather. 
Hitherto the grain of the developed plate has been dealt with, but, 
as Neuhauss pointed out, a grain can be seen before exposure, but with 
extreme difficulty. The author was most successful with a film deeply 
stained with cyanine, oblique monochromatic illumination, and a 
Zeiss objective of N. A. 1.40. The spotless white and: transparent 
emulsion will keep for several days unchanged by the direct action 
of light, a phenomenon which proves that the grain can not suffer 
reduction or blackening except with the help of some photographic 
reducer.¢ 
THE STRUCTURE OF THE PLATE IN PURE SPECTRAL COLORS. 
An examination of the sections through a pure or almost pure 
spectrum color shows different zones; first a laminated zone and then 
(below) an unlaminated zone. The structure depends on the thick- 
ness of the plate, the transparency of the emulsion, and the duration 
of exposure. Moderately thickly coated plates show the structure for 
about one-third or somewhat less of the total thickness of the gelatine, 
and the following parts may be seen: The limiting zone, which lies 
between the free surface and the first lamina, the Zenker lamine, and 
finally the intervals or spaces. 
The limiting zone in the blue and violet is very difficult to detect 
because it is so thin; in the red and orange, on the other hand, it is 
comparatively distinct as a very fine stripe which is almost free from 
grain formation, but the nearer one comes to the Zenker stripes the 
more distinct the grain. Even in the red the examination of this zone 
is not easy—sections immersed in water have a refractive index so 
near that of water that even by oblique illumination it is almost 
“This was first observed by Ltippo-Cramer and also by Neuhauss in 1903, 
therefore one can do away with the useless operation of fixing and the conse- 
quent reduction of the intervals between the laminz. Lehmann also does not 
fix the plates. 
