! 
244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
It is characteristic for white or gray, or all colors containing an 
admixture of white, that the limiting zone disappears. In its place, 
and in place of the first lamina, there appears a new dense dark film 
of great metallic reflective power, 5a and 6a. This lamina, sharply 
defined on both sides, contains large spherical metallic grains packed 
close together, and of a dark brown color. The general rule is that 
the more brilliant the white the more opaque and compacted is this 
region, which, if the plate is not intensified, is only a bright trans- 
parent yellow or light brown, with distinct spaces between the grains. 
This observation is important, for it proves that to obtain whites 
there must be (1) a metallic reflecting precipitate in the limiting 
zone, and (2) complete opacity of the first lamina, which combines 
with the limiting zone to form a morphological unit. The result 
is that nearly the whole of the incident light is reflected, and the few 
rays which do get through into the deeper parts of the plate can not 
produce interference. Behind the mirror zone there is a very fine 
interval, and a series of very dark, extremely thin, closely compacted 
stripes (fig. 5b). These stripes are never wanting, even if the white 
of the object is very pure. If the white is mixed with pink, cream, 
or bright blue, they are more numerous than in neutral gray. It is 
very significant that the distance between these lamine is extremely 
‘small—about the same as for violet and blue; sometimes a difference 
in thickness and separation can be seen, as though they were caused 
by light of differing wave lengths. They are so fine that it is difficult 
to see them in a plate that is not swollen in water. 
The author does not consider that these phenomena are contrary 
to theory. The thickness of the mirror zone on the surface of the 
gelatine is probably due to the combined action of the ultra-violet 
rays. The grain of the emulsion is too coarse to give regular periodic 
lamine, but only diffuse deposit. On the other hand, the blue and 
violet of greater wave lengths are registered, if only partly, whilst 
the comparatively coarse stripes which appear between the fine ones 
are perhaps the maxima for the long waves green, red, and yellow 
for which the chromatic sensitizing is least. 
The preponderance of violet in the image of white depends prob- 
ably on the rapidity of development. Then appears a phenomenon 
similar to that which is observed when a plate exposed for only a 
short time is exposed again for a much longer time. On development 
“Jf the mirror zone is formed more easily in slow plates, this is due to the 
fact, already mentioned, that these plates are specially sensitive to the shorter 
waves. The unequal behavior of the plate with the green, red, or orange, which, 
unfortunately, frequently happens, is due to the addition of the same quantity 
of erythrosine, cyanine, and glycine red to the emulsion. The whites are then 
frequently not pure, but tinged with red or yellow. : 
