LIPPMANN HELIOCHROMES—CAJAL. 249 
entirely to the first mirror zone and not to lamine. The following 
are also advanced in favor of the author’s views, and much against 
Lehmann’s: 
a. If the whites are rubbed with a pad dipped in alcohol till the 
mirror zone disappears, there appears first blue violet, although the 
opacity of the metallic particles is appreciably reduced when exam- 
ined by transmitted light. If the picture is still further rubbed till 
quite transparent, the white never appears when it is put in the 
benzole tank. The colors behave quite differently as they reappear. 
b. If a very thin plate is used so as to prevent the formation of 
the unlaminated zone, all the colors will be visible when the plate 
is looked at from the back, but wAzte is never seen. 
c. If a plate is left, without varnishing, exposed to the air for 
some months, the whites are the first to disappear, probably on ac- 
count of oxidization. This rapid alteration can be explained by the 
fact that the mirror zone, as already pointed out, lies absolutely on 
the surface of the gelatin. 
d. Everything which attacks the surface of the gelatin of the 
developed plate, such as washing, friction, deposition of mercury 
oxide on the sensitive film, etc., prevents the appearance of the 
whites, whether the plate is examined in air or benzole. 
e. In underexposed plates, if no color of the longer wave lengths 
green, yellow, and red has acted, nothing but a brilliant white is 
obtained on intensification, especially if slow-acting plates are used. 
On the assumption that two complementary colors, for instance, red 
and green or yellow and violet, have been registered, this formation 
of white is incomprehensible. 
f. Whites also appear on plates which have been exposed without 
the mercury mirror, and in which the lamine,are extremely thin. 
The white obtained by intensification is as brilliant as in pictures 
obtained under the ordinary conditions. 
g. White is also obtained by the intensification of pictures taken on 
nonorthochromatized plates. 
h. The examination of white in oblique light, that is, under the 
glass prism, shows, as already mentioned, not the least qualitative 
change, whilst all other colors are shifted toward the greenish blue. 
It should also be noted that whilst red, in passing into blue-green, 
misses the orange-red, yellow, and bright green, the blue only slightly 
shifts toward the violet. The result, which can be easily explained 
mathematically, is not in favor of Lehmann’s theory. If the white 
is actually formed by the action of two reflecting laminxw belonging 
to two complementary colors, as, for instance, red and green, it is 
not obvious why, in the shift of the red into blue-green and the green 
into dark blue, that is in the shift into two colors which are no longer 
