26 J. W. Draper— Distribution of 
1st —Case of the Compounds of Silver. 
Silver is the basis of the most important photographic sensitive 
substances. Its iodide, bromide and chloride, darkening with 
rapidity under the influence of the more refrangible rays, have 
mainly been the cause of the misconception above alluded to 
respecting the tripartite constitution of the spectrum. It is 
necessary, therefore, to determine what are really the habitudes 
of these substances. 
1.) If a spectrum be received on iodide of silver, formed on 
the metallic tablet of the daguerreotype, and carefully screened 
from all access of extraneous light, both before and during the 
exposure, on developing with mercury vapor an impression is 
evolved in all the more refrangible regions. This stain corre- 
sponds in character and position to the blackening effect which 
under like circumstances would be found on any common sensi- 
tive silver paper. It is this which has given rise to the opinion 
that the so-called actinic rays exist only i in the upper part of the 
spectrum. If, however, the action of the light be long con- 
tinued, a white stain makes its appearance over all the less 
refrangible regions. It Nee a point of maximum to which I 
shall again presently re 
But if the callie tablet during its exposure to the 
spectrum ts also receiving diffused light of little intensity, as 
the light of day or OF a s lamp, 1 it = ‘be found on developing 
that rs strikingly from the preceding. 
BKyvery ray that the prism can tenant from below the extreme red 
to beyond the extreme violet, has been active. The ultra-red heat 
lines a, f, y, are present. It must be ees in mind that the im- 
pression of these lines is a proof of proper spectrum action, 
and distinguishes it from that of diffused light, arising either 
from the atmosphere or from the imperfect transparency of the 
prism—a valuable indication. The resulting photograph shows 
two well marked regions or phases of action. On its general 
surface, which, having condensed the mercury vapor, has the 
aspect of the high lights of the daguerreotype, and forms as it 
were the basis for the spectrum picture, there is in the region 
of the more refrangible rays a bluish or olive- colored impres- 
sion, the counterpart of the result described in the foregoing 
paragraph. But in the region of the less refrangible rays no 
mercurial deposit has occurred, the place of those rays being 
depicted in metallic silver, dark, and answering to the shadows 
of the daguerreotype. This protected portion, which stands 
out in bold relief from the white background, reaches from a 
little below G to beyond the extreme red, and encloses the heat 
lines above named. They are in the form of white streaks 
Though I speak of them as single lines, they are in reality groups 
or perhaps bands. 
