592 
Photographs in Color,’ published in the Inter- 
national Annual of Anthony’s Photographic Bull- 
etin, XIII. (1901), 107. At that time and also 
many years later, the effects observed were at- 
tributed to an antagonistic action between light 
radiations from different parts of the solar 
spectrum. 
Many years ago, when collodion wet plates 
were mostly in vogue, there was consider- 
able discussion among photographers of the 
effect of exposing sensitized plates to diffused 
daylight, either before, during or after the 
usual exposure in the camera. Some claimed 
that such a supplementary exposure made the 
plates more sensitive, so that the camera time 
was materially shortened. The admission of a 
little diffused light through a hole in the camera 
was claimed to be advantageous in the same 
way. Others questioned the utility of the 
practice and the question was finally dropped 
and forgotten. 
There may have been a basis of truth in the 
contention of those who advocated the supple- 
mentary exposure, but it was not satisfactorily 
established at the time, There is less reason 
for skepticism now than there was in those 
days. Although not exactly in line with Pro- 
fessor Nipher’s work, the subject bears a close 
relation to it. 
More directly connected with the recent ob- 
servations is the work of M. J. Jansen, at 
Meudon, in the year 1880, when he was en- 
gaged in studying the solar radiations. In his 
original communication to the French Academy, 
published in Comptes Rendus of that year, he 
used the following language descriptive of his 
work: ‘‘I have the honor to inform the 
Academy of the discovery of a fact to which I 
have been led by my studies in the analysis of 
the light of the sun and of its photographic 
images. 
‘‘Mhis fact consists in this, that the photo- 
graphic images may be reversed, and pass from 
negative to positive by the prolonged action of 
the light which has produced them.”’ 
Ordinarily the exposures for negatives were 
about one-thousandth of a second, or when bro- 
mide plates were used, one ten-thousandth. But 
when the exposures were prolonged to half a 
second or a full second—increased 10,000 or 
SCIENCE. 
[N. S. Vou. XIIL No. 328. 
20,000 times—he obtained a positive picture in- 
stead of a negative. 
The investigations were continued and in a 
second communication to the Academy, also 
published in Comptes Rendus (Vol. XCI.), he 
made known some remarkable results. 
By varying the times of exposure he found 
an intermediate condition of the plate at which 
neither a positive nor a negative could be de- 
veloped. His conclusions may be briefly sum- 
marized here. With exposures of increasing 
duration he discovered six different successive’ 
conditions of the sensitive plate. These de- 
veloped in order as follows: 
1. A negative. The ordinary negative. 
2. A first neutral condition which blackened 
uniformly in the developer. 
3. A positive. 
4. A second neutral: condition, opposed to 
the first, which became uniformly lighter in the 
developer. 
5. A second negative, similar to the first but 
differing by the enormous amount of light re- 
quired to produce it. 
6. A third neutral condition, in which the 
negative of the second order had disappeared 
and was replaced by a sombre, uniform tint. 
These facts were established with different 
kinds of plates—tannin plates, gelatin-bromide 
and others. 
It is scarcely necessary to indicate the bear- 
ing of these observations on the results of Pro- 
fessor Nipher’s experiments. Does not the 
fourth condition suggest that if a plate in that 
stage were developed in a lighted room it 
would show a negative picture ? 
About the time of these observations of 
Jansen, considerable attention was being 
directed to the subject of reversals of the 
photographic image; but most of the literature 
deals with theories in explanation of the facts. 
Although the discussion was sufficiently in- 
structive and interesting, it does not seem to 
me that we are sufficiently acquainted with 
the chemical effects of light in photography to 
warrant much chemical theorizing in _ this 
particular field. 
RoMYN HITCHCOCE, 
NEw York, March 18, 1901. 
