THE STRUCTURE OF LIPPMANN HELIOCHROMES.2 
By S. R. Cagat. 
Lippmann’s heliochromes, as is well known, were founded on purely 
theoretical reasoning. They are interesting as a proof of the actual 
existence of ight waves. For this reason photomicrographs of sec- 
tions of the same are valuable, as proving how far the plate can 
register them. 
It is well known that the possible registration of these waves by 
a photographic plate was first pointed out by Zenker, and the results 
have therefore been universally called Zenker’s lamine. The point 
is that, though theory says that these lamine should exist, do they 
actually occur? Or, in other words, can one actually see them under 
the microscope? Obviously this is a difficult problem to solve, for we 
are close on the limits of microscopic resolution. Taking the case of 
green rays, their wave-length is 0.512 », a dimension which must be 
reduced to one-half, as the lamine are half a wave length apart. For 
the spectral green, then, we have to resolve an interval of 0.237 p 
(thousandths of a millimeter), which, according to Abbé’s formula 
at dL ; : 
for central white light, =~ requires a numerical aperture of over 
1.40, which is the practical limit yet attained with the Zeiss apo- 
chromats.? 
It is true that with oblique lighting we can increase the resolving 
power, d= = but, as Neuhauss has shown, this gives rise to diffraction 
phenomena, which obliterate the true lines, and may even cause 
reversal, as has been experienced by Senior and others. In spite of 
the difficulties, however, Neuhauss and Valenta alone have succeeded 
in obtaining excellent photomicrographs.° Senior’s results,’ as 
pointed out by Neuhauss, are far too thick, and are really diffraction 
@ Reprinted, by permission, from the color photography supplement of the 
British Journal of Photography, for August 2, September 6, and October 4, 1907. 
>The objective of N. A. 1.60 with monobromonaphthaline immersion can not 
be used, as one dare not imbed Zenker’s lamins in this solution. 
* When this was written (June, 1906), the author says he was unaware that 
Dr. Hans Lehmann had also obtained photomicrographs of sections of Lippmann 
heliochromes. (See “B. J.,” November 30, 1906, p. 946.) Dr. W. Scheffer has 
also obtained photomicrographs of Lippmann plates.—Ebs., “ B. J.” 
@* Photography,” January 3, 1902. 
