242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
impossible to see it. This is probably the reason why Neuhauss ex- 
pressed doubts as to its existence. The author was enabled to see it 
by treating the film with an aniline dye, such as aniline blue, which 
is insoluble in water or by coating it with colored varnish. Under 
these conditions, thanks to the colored film, it can be plainly seen 
(fig. 1, p. 246). Its thickness in the swollen plates is about half an 
interval, but varies considerably, which may be caused by unequal 
expansion of the gelatine, and also to the varying thickness of the 
first lamina. 
These observations confirm, at least in principle, the often-observed 
fact, that the surface of the gelatine forms the first interval, and that 
the reflecting surface of the mercury is thus in immediate contact with 
the gelatine during exposure.? 
As the limiting film is only a fraction of a wave-length thick, it 
is easy to understand why the lhght reflected from the surface inter- 
feres with that from the lamine, and why a prism is necessary or 
the heliochrome must be placed in a cell filled with benzole or xylol 
to eliminate this surface reflection. 
The Zenker lamine consist, as required by theory, of a metallic 
precipitate which is thicker in the middle than the sides. It must 
not be overlooked that in the dry plate the lamine are very close 
together, and that they have great density and considerable reflective 
power. In the unintensified plates the color of the grains is bright 
brownish yellow, in the intensified gray or coffee brown. 
The number of the laminz differs considerably. In many cases 
it varies, as pointed out by Neuhauss, between four and six, and 
depends on the intensity of the light, the duration of the exposure, 
and the transparency of the gelatine. Generally, the author thinks 
that there are more in brilliant pure colors, as in the solar spectrum, 
than in the mixed colors of natural objects. He has some spectre 
showing thirteen and more lamine, which reach to the glass and show 
the colors from both sides. The same effect has been met with in some 
histological heliochromes.? There are exceptions, and as a rule the 
number of laminve is only five, six, or eight. 
The thickness of the laminz is everywhere the same, as is also that 
of the intervals. Their intensity and the sharpness of their edges 
decrease the farther they are from the surface of the plate. This 
“This fact is opposed to Rothé’s assumption that the laminze are due to the 
light reflected from a film of air between the mercury and the gelatine (‘ Compt- 
Rend.,’ 1904, pp. 565-567). If this was so the reflection of the incident light 
would take place from a substance with lower refractive index than that of 
gelatine, therefore there must be formed a maximum and not a minimum on 
the surface of the gelatine. This is never the case with correctly exposed un- 
intensified plates. 
> Professor Cajal uses this process for obtaining heliochromes of histological 
and pathological sections.—lbs. “ B. J.” 
