LIPPMANN HELIOCHROMES—CAJ AL. 251 
that the plate will reflect only a small part of the incident hight and 
also allow the dark background of the asphalt on the back of the 
plate to shine through. Naturally, the color will be much darker 
the paler the lamine. Dark colors also appear very stable when the 
plate is rubbed, a fact which is easily understood when one bears in 
mind the extraordinary transparency of the lamin which take part 
in the interference. 
BRILLIANCY AND PURITY OF THE INTERFERENCE COLORS. 
Everyone who has worked at all with the Lippmann process will 
have observed the great differences in the purity and brilliancy of the 
colors. Some very transparent plates reproduce the whole of the 
spectrum in brillant pure tones; other emulsions give all the colors, 
but dead and impure; others again as though covered with a gray 
or white fog. Some fairly sensitive plates, which give otherwise 
good colors, convert the white into gray, violet, or cream; others 
again give certain colors, usually red, orange, and yellow, fairly well, 
but are totally wanting in green, blue, and violet. 
In order to understand these phenomena one must bear in mind 
that Zenker’s exact theory is only carried out under defective condi- 
tions, due chiefly to the special nature of the photochemical actions. 
The lamine are not absolutely smooth and sharply defined, nor are 
they everywhere of equal thickness, also they do not possess that uni- 
form perfect transparency which theory requires, so that all may take 
part in the interference of the incident white light. 
The brilliancy and intensity of the interference colors depends, at 
least so it is generally assumed, on the perfection of the lamellar 
structure of the plate, and the purity and brilliancy of the colors is 
greater the greater the number of the reflecting lamine. Broadly 
this view is correct, but theory does not coincide with practice. The 
author states that many of his pictures of great brilhancy and truth 
possess only three or four especially brilliant and correct reflecting 
lamine, whilst others with ten or twelve regular distinct laminz 
gave less bright pictures. The brillancy of the colors thus depends 
not on the quantity, but the quality of the lamine and intervals. 
From some hundreds of very careful observations the author comes 
to the conclusion that in most cases the color is due to the reflection 
and interference of light from the two uppermost lamine. The 
deeper-lying ones have very little to do with the formation of the 
colors; in the first place because they receive but little light, and 
therefore can only reflect little; secondly, because they have not 
sharp limits and are not separated by perfectly colorless intervals, so 
that the ight can not be properly analyzed, but only diffused; and 
