LIPPMANN HELIOCHROMES—CAJAL. 259 
there are some fine closely contiguous stripes, which probably corre- 
spond to the short waves of the visible spectrum. 
5. The colors mixed with white show with their own lamine a 
thin surface film, filled with a metallic precipitate, the mirror zone. 
6. In certain cases colors mixed with white show two kinds of 
lamine, large stripes far removed from one another, which belong 
to the long waves of the predominant color, and one of two fine pale 
films corresponding to the shorter wave lengths. 
7. The interference phenomena, through which the colors are 
produced in Lippmann heliochromes, can be ascribed actually to the 
action of the rays reflected from the first and second lamine. The 
others have only a faint, but, to a certain extent, an intensifying 
action. Pure spectral colors are an exception in their formation; if 
the metallic precipitate is quite transparent, the deeper lying tones 
may also act. 
8. The good rendering of the colors is principally caused by the 
correct limiting and perfect transparency of the upper lamina, as 
well as the normal value of the intervals. All causes, such as long 
exposure, overdevelopment, incorrect intensification, etc., which 
upset the ratio of the two first lamine as regards intensity and 
thickness, or such things as fixation and damp, which affect the size 
of the intervals, alter the true colors and cause false tonalties. 
From this it 1s obvious that the greatest difficulties of the Lippmann 
process are as follows: a, the distance of the individual lamin pro- 
duced in the plate by the colored hght during the exposure must be 
strictly kept, in spite of the contracting action of fixation and the 
expanding action of intensification; 4, too little transparency and 
too great thickness of the first lamina must be avoided, although a 
certain opacity is esential for the correct reproduction of the bright 
tones. By careful, clean work the perfect balance of these two 
opposite requirements must be fulfilled by workers in interferential 
photography. | 
