on Silver Iodide and Bromide. 273 
opment could be obtained on the silver bromide paper, after 
exposures of various lengths up to an hour and a half of bril- 
- lant sunshine (from 12.15 to 1.45 p. M., Jan. 30, 1875, bright 
sunshine on snow). 
n the contrary, silver iodide gave, after 20’ exposure, a de- 
veloped image showing some detail, and with 40° exposure, a 
faint, direct image, visible without development. 
With three red plates, having a ining wave-length A 605, 
there was necessarily a further eet e age ae of illumination. 
Nevertheless, silver iodide gav ) minutes’ exposure 
(middle of the day, bright Aaaies on ara sie a; iste b> 
faint, and with four hours’ exposure, a full image. Feb. 
three hours’ exposure gave an image insets: ‘ieudetiele Fi 
tail. Feb. 5th, same result. These were of course all devel- 
oped images. 
The corresponding bromide papers, receiving identical ex- 
posures under the same glasses, side by side with the iodide, 
absolutely failed to develop anything. These developments 
were prolonged for several hours, in order that the faintest 
traces, if present, might render themselves visible. But in no 
case did silver brom nide, when exposed under the three red 
glasses, show the faintest trace of any image, even with a four 
hours’ bes want That silver bromide is not ot destitute 
pears, therefore, pb that both silver iodide and silver 5 eH ° 
prepared on r, with excess of silver nitrate removed, are 
distinctly senaitet to red light, and that silver iodide is, under 
these conditions, at least ten times as sensitive as silver bromide. 
Yetitow Rays. 
It is by far more difficult to isolate yellow rays than either 
red or green, because almost all media that transmit yellow 
rays also transmit red, and many also transmit green. The 
yellow Agee that is found in commerce _ through the whole 
Spectrum, except the extreme violet end, and for a time I 
thought that the isolation of the — et by colored glass 
would be impracticable. I finally succeeded very well by 
combining a deep brown glass with a dark green. The brown 
glass Pic AR the sero e and red, absorbing the rest, and 
