Inexpensive Screen for Monochromatic Light. ByJ. W. Gifford. 1G5 
too broad, or it lies among the rays less refrangible and less suitable 
for photography. There is also great loss of light. 
While at work on the various aniline dyes in connection with 
their photographic effect on the salts of silver when these are exposed 
to the light of the solar spectrum, it came to my notice that the 
absorption spectrum of benz-aldehyde green, commonly known as 
malachite green, was a very remarkable one. 
We have two very broad dark absorption bands (fig. 2), one 
which has for its centre a point slightly less refrangible than line D 
in the solar spectrum, and extending from about B to E, with an 
aqueous solution of mean strength. A second band, having Gr for its 
centre and extending from a position slightly more refrangible than 
F to the calcium lines These absorption bands, to which I 
shall not again refer, leave us bands of light at three distinct points, 
viz. (1) From A to B, a narrow red band of low intensity ; (2) from 
E to a little beyond F, a rather wider, intense band ; (3) from HxILj 
to M, a feeble, invisible band, the position of which can only be ascer- 
tained photographically. The band of light from E to F is the one 
to which I wish more particularly to direct your attention. It is in 
this region of the spectrum that blue graduates into green, and we may 
imagine a central point in the band which is neither blue nor green. 
Now by dissolving the dye in different liquids, the position of this 
blue-green band may be modified to a considerable extent. I need 
not dwell on the fact that in common with most substances in solution 
which show absorption phenomena, these bands, and especially that 
one with which we are most concerned, will be widened by further 
dilution with the same solvent. But by the use of different solvents 
the position of the blue-green band in the spectrum may, within 
certain limits, be so shifted that a line drawn through its centre shall 
occupy a place more refrangible, or less refrangible, as the case may 
be, and in this way, and by the addition of other substances, the red 
band, as shown in aqueous solution, may be considerably reduced in 
intensity, and the faint band in the ultra-violet entirely removed. 
Thus, by solution in glycerin the blue-green band is narrowed 
and shifted, so that it almost exactly fills the space between E and F 
when a given thickness or strength of solution is used, and, although 
narrowed, it is much brightened. At the same time the red band 
becomes much fainter, so that even for purposes of focusing it may 
be neglected. 
If glycerin-jelly be used for the solvent, the blue-green band 
occupies a place midway between that of the same band in aqueous 
and glycerin solutions, transmits almost as much light as glycerin, and 
photographs more rapidly. 
A solution in cedar-oil may be made without great difficulty, and 
in this case the band is even less refrangible than in the case of 
glycerin, and broader, and there is slightly more light, less being lost 
by reflection from the glass surfaces in contact with it. 
