LIPPMANN HELIOCHROMES—CAJAL. 253 
3. As already stated, the brilliancy of the color is independent of 
the thickness of the plate and number of the lamina. Very brilliant 
colors are seen in quite thin plates of 4 to 5 pr. thickness. 
This refers mostly to colored objects, in which there are usually 
compound colors. With spectra photo-micrographs of anatomical 
preparations or, briefly, when pure or almost pure waves act on the 
plate, the deep lying lamine are almost as well formed as the surface 
ones. Naturally in such cases friction only destroys the colors with 
the fourth or fifth lamina. 
ANALYSIS OF WHITE PLATES CAUSED BY EXCESSIVE INTENSIFICATION. 
The above results of the author’s researches on the whites elucidate 
a phenomenon which is often observed before or after fixation, when 
a plate is intensified with perchloride and amidol, plus sulphite. 
It has already been stated that the grains become larger, and, there- 
fore, closer together. Consequently, the reflective power, particu- 
larly of the first lamina, which is most easily attacked by the reagents, 
is increased. 
So long as the grains of the first metallic film possess a certain 
transparency the color does not markedly alter, as part of the inci- 
dent light reaches the second lamina and is reflected back. If, how- 
ever, as is generally the case with a second intensification, the first 
film loses its transparency almost entirely, then the ratio of reflect- 
ive power of the first two films is altered, as that of the first pre- 
ponderates. The result of this is that the color presents a dirty white 
appearance, and the want of transparency is greater the thicker 
the grains of the first lamina become. With great intensification the 
colors completely disappear, especially in the fully exposed parts, and 
the picture appears as though covered with a milky fog. 
Figs. 7, 8, and 9 show the appearance of a section through almost 
pure green before and after intensification. Before intensification the 
laminz are pale and fine-grained, and the metallic precipitate is 
absolutely wanting on the surface (fig. 7). Therefore the light can 
penetrate to the second and third film, and their analytical and 
reflective actions are added together. It is quite different in fig. 9, 
which is a section through the same color after two intensifications. 
All films, especially the first, act like a white-producing mirror—that 
is to say, they contain extraordinarily coarse grains and have lost 
the best part of their transparency. Moreover, it can be seen that 
each film has become distinctly thicker. The limiting zone has given 
way to the mirrror zone. Fig. 8 shows the same color with one inten- 
sification. 
The practical result of these researches leads one to formulate the 
rule that Lippmann photochromes should be intensified once to give 
