MAPPING THE LEAST REFRANGIBLE END OF THE SOLAR SPECTRUM. 655 
which it exists. The bleached dyes partake of the nature of this organic matter, and 
we are forced to conclude that a state of oxidation favours this disposition. From 
exposure of the dyes in different media to the action of the spectrum it seems that the 
bleaching is due to a state of oxidation of the dye. If a film containing silver bromide, 
and dyed, were exposed in hydrogen to the action of the spectrum, the density curve 
was that due alone to the action of silver bromide, and the dye did not affect it, 
excepting so far as it acted as a screen to prevent the full intensity of the light falling 
on the bromide. 
The dyes which are most active are those which are known as yielding fugitive 
colours; a permanent colour produces no effect beyond the above screening the light 
from the silver bromide. It should be remarked that when a film containing finely- 
suspended matter is dyed the change effected on the dye is much more rapid than 
when a continuous film is acted upon, for the dye surrounds these very small particles, 
and thus a large surface of it is exposed to the air, and renders oxidation comparatively 
easy. As a result of these experiments I can confidently state that in no case did the 
addition of the dye cause any chemical effect to be produced by the rays below A of 
the solar spectrum, nor has Vogel claimed that they do. I am aware that in the 
Proceedings of the Ptoyal Society 4 ’" Major Waterhouse has stated that by staining the 
film with turmeric he obtained evidence of the existence of lines in the solar spectrum 
a little less refrangible than A, but it must be observed that the lines so shown are, 
except in one instance, “ reversed.” That is, absorption lines appear as opaque on the 
transparent body of the spectrum instead as of a normal character, viz.: transparent on 
an opaque body. This reversal is a matter to which I have referred in the Proceedings 
of the Royal Society,! and is dependent on a different action entirely to that which I 
am now considering. 
Preparation of a film sensitive to the infra-red region. 
My earliest endeavours were directed to extending this action of organic matter, so 
that sensitiveness of the compound might be obtained in the ultra-red regions. By 
weighting the molecules of the silver bromide with gum resins, the spectrum was 
impressed considerably below A and the absorption lines were unreversed. Measures 
of the heating effect of the solar spectrum on lampblack, as shown by the thermo-pile 
and Sir J. Herschel’s well-known researches, showed that the lower limit of the 
prismatic spectrum had not been reached; it therefore seemed advisable to search in a 
different direction for a more sensitive compound. The salts of silver still seemed the 
most feasible to work with, and more especially the bromide, and efforts were made 
to obtain this compound in a different molecular condition to that generally found. 
I need not detail the different methods of preparation of this compound in collodion 
that were carried out. In some cases I obtained it in a state which when viewed in a 
* Yol. xxiv., p. 186. 
f Yol. xxvii., pp. 291 and 451. 
4 P 2 
