JoLtyY—On the Sensitiveness of the Photographic Dry Plate. 228 
longitudinally by poultices, as before described. Upon this the 
spectrum was formed, from an electric arc, of such dimensions as 
nearly to extend the full width and length of the plate. 
When an isochromatic plate (Edwards’) was exposed for a few 
seconds to the spectrum and developed, it was found that the warm 
half of the plate showed the usual appearance: strong over the 
violet and blue, with a weak region in the blue-green (between F 
and HK), and again a vigorous action in the green and yellow-green 
(between E and D), fading off gradually to the orange-red. This 
green-yellow sensitiveness is well known to be especially the result 
of the action of the dye. 
On the cold half of the plate, over the violet and blue regions, 
there was an equal, or very nearly equal, density to that obtaining 
on the warm half; but all beyond, beginning at the weak region, 
K-F,, and extending to the limit of sensibility, there was a most 
marked and striking loss of sensibility. Over the weak region 
there had been no action, or very little; the dense band marking 
the green-yellow was weakened and narrowed, showing, however, 
no shifting, and the yellow and orange again were almost without 
density. 
Some ordinary gelatino-bromide plates (Wratten’s and Cadett’s) 
were next tried. In the case of these the hot and cold regions did 
not exhibit so marked a difference. The warm extended, indeed, 
further beyond F towards H, and there was a slightly inferior 
density all over the cold spectrum. I am not perfectly sure if this 
last is a real effect, for some stray light and fogging introduce 
uncertainty as to whether this may not be—in some degree at least 
—due to the inferior sensibility between Hand F. For such rays 
in the stray light would be inactive over the cold, and active over 
the warm halves of the plate. The most marked effect is, how- 
ever, in this case also towards the less refrangible rays. 
It remains to consider how these results may be interpreted in 
connexion with theories of photo-chemical action and the action of 
the special sensitizers. 
In the first place, we might consider that the deprivation of 
heat simply affected the fundamental silver bromide molecule, 
rendering it less resonant to the long wave-lengths, and hence 
affecting the photo-reduction on the plain gelatino-bromide plate 
in so far as this is ordinarily sensitive to long wave-lengths, and 
SCIEN. PROC. R.D.S., VOL. VII., PART III. R 
