2 O. N. Rood on Photometric Experiments. 
and when once effected of course remains undisturbed by fluc- 
tuations of the flame. In addition to this it will also be observed 
that a more delicate screen, constructed, however, on the general 
principles indicated in the first part of this paper, has been em- 
ployed, and these modifications, taken in connection with cer- 
tain precautions described below, have given the results an 
neeinlon superior to that attained in the experiments on silver 
mirrors 
keg Som! of the photometer with a single gas flame for ex 
ts on the amount of light transmitted by plates of various 
substances 
The source of light is a small gas flame at L, about an inch 
in height, the gas flowing as before from 
lain circular opening. A portion of its M. 
ht, LA, directly nose Se the screen 
Ss, while another portion N A is reflected 
on the screen by a iabie: $ mirror, one 
protected by a coating of copper being 
selected : this equalizes ve light on both 
sides of the bare “spot” inS. A thir 
portion of the light from L falls on M, a 
mirror like that just mentioned, and there 
is reflected perpendicularly through FP, 
inati he 
ground glass. It will be observed that 
the path pu by the light in this lat- 
s the distance LM+MG: by 
the © graduation sess the instrument MG 1s 
ly measu P is known; also 
PL: hence fe ave the means of calou- B 
lating the distance ML, which is to be 
added to MG, giving the distance — Tv 
ed. As these calculations involve so 
stor, I constructed for my aeeutacnt. a table by which the 
easured distance GM, is readily converted into the total dis- 
— The photometer was the same as that described in the first 
. yar of this paper, the following changes having been intro- 
uced: the “spot ” on the screen was made much Srnadled being 
only about zz of an inch in horizontal diameter, with a length 
about three = as great; it was observed by a small telescope 
3 ix diameters. reduction in the size of the 
? 
iii 
it ina mata beyond reproach. 
t” ret sesitlered it it possible to illuminate the ground surrounding : A 
