O. N. Rood on Photometric Experiments. 9 
a lap i 2 deviation even after the main portion of the 
ent has ceased to perceptibly deflect it. 
ts no case did I push a gestae far enough to gather 
I give now the results of careful sets of experiments on the 
amount of light transmitted by two different samples of crown 
glass: in each case the results of four independent trials are 
given, each trial being worked out with the aid of seven double 
compensations. In the first case the thickness of the glass was 
15 millimeters, the index of refraction 1°5236, and sig for 
the effects of internal reflection, it should, according to theory, 
have transmitted 91-736 per cent of the light falling on it. Ex 
periment gave: aes 
91-440 
The difference ‘296 being hardly larger than the nec 
error connected with the method of making the detensaiiaane 
In the second case the index of refraction was 15225, the 
thickness 1°677 millimeters, and by theory it should have trans- 
mitted 91°763 per cent of the light falling on it. Below are the 
actual results obtained: 
90886 
90-948 
90892 
91895 
91155 
ee pape — of 500 per cent, or 51; of the whole 
is almost equall eatiataotory, and these experiments 
ig: T "chink, that the reflecting power of glass with the above 
index of refraction, conforms in the closest manner to the 
tions of theory. Elaborate experiments were also made wit. flint 
glass, quartz and calc-spar, but I suppress the results, as it after- 
