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LIPPMANN HELIOCHROMES—CAJAL. 943 
fact, as will be seen later, is very important. Figs. 2 and 8 show 
that the first lamina is the most distinct, and, as a rule, more sharply 
defined on the edge. Then follows the first interval, which is the 
purest and most colorless—that is, the freest from silver—of all; 
then the second lamina, dense and sharply defined; then the second 
interval, which is almost as clean as the first. Behind these the con- 
trasts between the lamine and intervals are less distinct as the inter- 
vals become filled with precipitate, till the finai region is reached, in 
which the lamine disappear, as does also the silver precipitate 
(fig. 1d). 
As will be seen later, the relative intensity of the first lamine varies 
according to the duration of exposure and the degree of intensifica- 
tion. In normal plates the two first lamine are practically the same 
intensity and thickness; in overexposed plates the first lamina, in 
consequence of photo-chemical fatigue, is weaker or disappears alto- 
gether. In this case the second or third are the strongest. | 
The film without laminz varies considerably as regards thickness. 
In very thin plates it is almost or entirely wanting. In moderately 
thick plates, as in figs. 1 and 2, it may be two-thirds to one-half of 
the total film. Asa rule it is without silver grains, though here and 
there some may be seen which possibly correspond to oversensitive 
bromide of silver. If the exposure is too long, or the plate is developed 
too much, this region is filled with a fine yellowish or light brown col- 
ored precipitate of coarse particles. This very frequently happens 
in the pure red or yellow. The appearance of the section through the 
other colors is, independent of the function of the wave length, prac- 
tically the same. Fig. 2 shows a section through the blue at A 0.475 p. 
The extraordinary thinness of the limiting zone and the comparatively 
great fineness of the laminz will be noticed. In many sections the 
author thinks that there are less lamine in the blue and violet than 
in the more refrangible colors. It is difficult, however, to follow these 
fine lamine, and it may be merely a case of coincidence (figs. 2 
and. 12). 
THE ANALYSIS OF WHITE AND GRAY. 
These are two most important colors, and from a careful consid- 
eration of all the literature on the subject and the study of many 
sections the author comes to the conclusion that the formation of pure 
white is produced by intensification of the images. 
Without intensification—that is, without the artifical production 
of coarse grain—it is not possible to obtain pure whites, for these 
require a closely compacted, opaque film with metallic luster. As 
will be seen from fig. 5, the whites consist of three regions, the mirror 
zone, the laminated zone, and that which is characterized by diffuse 
reflection the rear zone. 
