LIPPMANN HELIOCHROMES—CAJAL. 245 
only the longer-exposed picture is seen. Perhaps, also, the greater 
attraction of the violet maxima for the developer comes into play, 
and the places corresponding to red and yellow scarcely act. There 
appears, then, the well-known action of contrast, which is frequently 
observed on ordinary plates, namely, an extremely bright margin 
round a vigorously developed place. 
From this it would appear as though the formation of white on 
those parts of the plate affected by ight of every wave length is not 
due to the admixture and fusion of the reflective action of many 
different lamin, as assumed by Lippmann, but exclusively to the 
reflective powers of a dense, opaque, dark surface film, on the opacity 
of which the brilliancy of the color depends. Consequently, neither 
the fine lamine within the plate nor any interference phenomena 
(since the density of the mirror zone makes this impossible) have 
anything to do with the appearance of white. 
That the author is correct is proved by the following phenomena: 
1. If the white places are rubbed, their brilliancy decreases with- 
out color appearing; only when the mirror zone is completely re- 
moved does white disappear and blue or a more or less dark gray of 
violet or bluish tinge appear. In the first place there always appears 
a greenish blue tone, the formation of which, as shown by micro- 
graphic examination of the rubbed parts, must be ascribed to the 
mirror zone becoming thinner. As soon as this zone is removed there 
appears a dirty indefinite violet, which persists till the plate becomes 
quite transparent. This last fact proves that the whites are caused 
by the action of the violet rays. 
2. Oblique illumination of Lippmann heliochromes produces, as is 
well known, a shift of the colors toward the more refrangible part 
of the spectrum. Orange-red becomes yellow, green, blue, and so on, 
and this shift is the more distinct the greater the angle of incidence. 
This change of the picture in oblique light depends on the laminated 
structure of the gelatine, and is easily explained by the increase of 
path, whach the waves of shorter wave length than double the inter- 
vals must traverse. Inclination of the plate to the incident light 
produces no change in white, a certain proof that this color does not 
depend on laminar formation. 
3. Neither varnishing the picture, nor slight swelling, nor testing 
in a benzole tank have any influence on pure white, which is thus 
sharply differentiated from other colors. This is also an indirect 
proof of the absence of the limiting zone above the mirror zone. 
Impure whites or grays will naturally alter in tint under a prism or 
oblique incident light. 
