252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
thirdly, because by development and intensification the two upper 
films far surpass the others in reflective power. 
The most important experiments which support these views are 
simple and easy to interpret. 
1. As already noted, and as Neuhauss states, rubbing the dry plate 
with a pad dipped in absolute alcohol causes tlie colors to shift to- 
ward the violet. The red becomes orange-red, then yellow, then 
green, and finally blue and violet, and these colors persist for an un- 
limited time in the dry plate, or if it is immersed in a benzole tank. 
This is explained by the erosion of the first lamina. If the friction 
is continued the violet disappears and the original color appears. 
This will occur once or twice, but it is then so dark-and dead that its 
action on the tint of the underlying part of the plate is almost nil. 
Friction beyond the fourth lamina produces complete disappearance 
of the original color. Friction is therefore an excellent method of 
studying the functional action of individual zones. 
The above phenomena are quite clear on the assumption that only 
the first two laminz, or perhaps also the third after intensification, 
take part in the formation of the color. Asa matter of fact, rubbing 
with alcohol very slowly reduces the thickness of the first lamina, so 
that the distance between its surface and that of the second lamina is 
reduced, and therefore it has all values between the half wave length 
of the original color and violet. If the first lamina is completely 
removed the surface of the gelatin is formed by any plane which is 
parallel to the first lamina. In this case the distance between the two 
reflecting planes, that of the surface of the plate and the second lam- 
ina, already smaller than the half wave length of the violet, and there- 
fore no color can be produced. If the third lamina is not sharply de- 
fined and does not possess sufficient reflective power, color definitely 
disappears. In certain cases, however, the third and even the fourth 
lamina are effective, and then we have the original color, but very 
dark and dead. Thus the color of an orange, after it has disappeared 
through removal of the first lamina, appears, through interference 
between the second and third films, brownish or dark orange, when 
examined in the benzole tank. 
2. The correctness of these views is shown by an examination of 
thin sections obtained by hacking the gelatin crisscross fashion with 
a scalpel. ‘Treated thus, a red which had shifted into green showed 
that the first lamina only had become thinner; the appearance of the 
blue was coincident with its almost complete disappearance, and the 
reappearance of the red, assuming that the second and third lamin 
were not damaged, took place when the friction was continued to the 
second interval. The original color finally disappeared with the 
destruction of the second lamina. 
