by Carbon Dioxide. 193 
pletely closed. Concentric with the trough, and likewise 
supported by the stand, was a silvered glass concave mirror of 
one meter focal length and 16™ in diameter, forming as it were 
the bottom of the tube when the latter was in place. By this 
arrangement the tube could be readily removed, while when in 
place it was stopped in a perfectly air-tight manner at the lower 
end, and the direction of the axis of the mirror could be adjus- 
ted by the foot screws without any motion being communicated 
to the tube. e upper end was left open, and reached nearly 
to a wide flat board, firmly supported in a horizontal position 
tion of two rectangular plane mirrors, likewise of silvered glass, 
Joined at right angJes like a letter V, with the lower angle over 
the middle of the hole and the reflecting surfaces facing out- 
ward, so that each made an angle of 45° with the vertical. 
The line joining their centers was therefore horizontal, and was 
adjusted to a height of 6™ above the upper surface of the 
board. In front of one of these mirrors was a slit, consisting 
t 
of a hole 1™ square in a copper plate faced on the side nex 
mirror or 4 meters, 3-4 meters of the distance being traversed. 
Within the tube. To facilitate the adjustments the slit was 
