on Silver Iodide and Bromide. 275 
Near these limits the light was extremely faint. To ascertain 
the rays of really effective strength transmitted by this com- 
bination of plates, another measurement was taken, resulting as 
follows 
Effective limit-toward Yed end,. ---- 2.22 4... A SBD 
Effective limit toward blue end, -.....-.--..._-- A517 
The portions of the band which lay between A 569 and A581 
on the one side, and A497 and 1517 on the other, were so ex- 
tremely faint as to have no ihn agency in affecting the 
result. 
It will be observed that even the limit of the faintest rays 
does not extend as far as the solar line F; the blue is therefore 
absolutely excluded. Toward the yellow side, even the faint- 
est rays do not extend so far as the sodium double line D and 
the effective aye terminated at A569. The yellow is therefore 
virtually exc 
Result. With two dark - ‘green glasses (A 488-601). A pow- 
erful image was developed on silver iodide after three minutes’ 
exposure. On silver bromide the same exposure gave faint 
traces of an image only. The exposure of an hour and a half 
failed to produce on silver bromide as strong an impression as 
did the three minutés on the iodide. 
An exposure of forty minutes gave on silver iodide a plain, 
direct image. With ninety minutes, no direct. image was pro- 
duced on silver bromide. “Other exposures and degrees of ex- 
With three dark green glasses (extreme limit 2497-581, 
eee 4517-569). Silver iodide gave the following results : 
a half minutes, faint image developed. Six minutes, 
Setiaot: fifteen minutes, strong; thirty, very full exposure ; 
three hours and a gre com — plsaace 
Silver bromide aa and six "minutes, nothing ; 
fifteen, faint trace ; shee, a little stronger; three and a half 
hours, moderatei st rong. 
Comparing the above: AgBr with fifteen minutes about the. 
same as Agl with two anda half: AgBr with thirty minutes 
not sal so aba: as AgI with six: Se whee with three and a 
Agland Aoi together. —Although the scope of this investiga- 
tion was directed to the action of pure AgI and pure AgBr, a few 
