LIPPMANN HELIOCHROMES—CAJAL. 255 
the path of the rays reflected from the surface and from the interior 
of the plate is enlarged, the same wave length, even with normal, or 
almost normal, illumination, which produce the lamine, will not pre- 
dominate, as will light of a greater wave length. 
One of the most unpleasant and most frequent occurrences in Lipp- 
mann’s process is the transition of the blue and violet into white. 
This change is due, not to a narrowing of the intervals, but only and 
alone to their lessened transparency, and especially that of the first, 
which then acts as an opaque screen. It is thus quite immaterial that 
the laminz and the deeper-lying ones are sharply defined, or that 
the top one remains intact, the waves of light can not actually pene- 
trate to the lower lamina, and therefore can not produce interference. 
In plates examined without a prism and without the benzole tank, 
this trouble often appears if the blue shows well, because the limiting 
zone, as is easily seen, is the more troublesome the shorter the wave 
length of the lght. 
If the tank does not remedy this fault, one can reduce the plate 
so as to enable the light to penetrate into the depths of the film. Asa 
preventive the use of light screens has been suggested to reduce the 
energetic action of the shorter spectral waves. Such screens have 
been used by all experimenters, and especially by Neuhauss and Leh- 
mann, with good results. The author uses a weak solution of aniline 
yellow with some erythrosine in collodion on the back of the plate; 
the use of the screen, which is rather expensive, is thus avoided. Also 
a screen absorbs a great deal of light, and if not of first-rate quality 
detracts from the purity of the pictures. 
FALSIFICATION OF THE COLORS THROUGH DAMPNESS OF THE PLATES. 
Similar falsifications of the colors appear in the use of too dry 
plates in damp weather. The correctly obtained and fixed lamine 
become considerably farther apart by absorption of atmospheric 
moisture, and the oft-noted fault of a shift of the colors toward 
the red is seen, and green becomes yellow, and yellow orange or red, 
and so on. In order to obviate this fault the plate should be brought 
into hygrometric equilibrium with the air. A somewhat dangerous 
remedy is reducing the grain of the lamine with a reducer.¢ 
@The reducers and especially dilute potassium cyanide solution, when care- 
fully used, restore the colors of overdeveloped or damp plates. But not only do 
the whites suffer severely, but after some time the grain bleaches very much, 
and the picture becomes worse. The author has therefore entirely given up 
the use of reducers. Only in individual cases does he use it locally to restore 
the blues and violets. This retouching is done on the wet plate with a fine 
brush dipped in weak potassium cyanide solution. 
