the radiant heating effects from terrestrial sources. 197 
being painted with indian ink, and the other coated with 
white silk pasted on, when exposed, might thus be compared 
with those through a glass screen. I first tried the experi- 
ment by placing the bulb in the focus of a spherical tin 
reflector about six inches diameter ; by this means the source 
of heat could be placed at a sufficient distance to preclude 
any effect from the glass screen. 
(18.) The experiment was again varied by placing a large 
opake screen before the instrument, in which was an aper- 
ture through which one bulb might be exposed. To this 
aperture a piece of glass could be applied ; each bulb was 
presented both with and without the glass. 
(19.) In all these experiments it is evident, that any heat- 
ing effect arising from the screen, would tend to diminish 
the ratio of the black and white effects ; and this not being 
allowed for in the statement of the result, the difference be- 
tween this ratio and that of the exposed effects will be in 
reality greater than appears. 
The result are comprised in the following table. 
Lamp. Bulb in the focus of a reflector. 
Coatings white silk, and indian ink. 
Experiments. 
Screened. 
Exposed. 
White. 
Black. 
White. 
Black. 
1 
5 
8 
11 
15 
2 
4 
9 
13 
15 
3 
6 
11 
12 
14 
Mean in 30 sec*. 5 
9-3 
12 
15 
