O. N. Rood on Photometric Eaperiments. 149 
w to the joint support of the arm, A, and of the 
mirror to he experimented on. = consists of a block of brass, 
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B, four inches square (see figure 2, which is one-third of the 
size); its under surface is cut in such a way as to fit the 
two parallel iron bars and to slide on them easily but steadily, 
and this foundation block is farther provided with a vernier, 
to read off the distances on the millimeter scale. C isa eradu- 
ated circle six inches in diameter, and is provided with a clam 
atd. The hollow massive cylinder, ¢, supports the arm A, an¢ 
also carries the axis of the support of the mirror. The screws, 
1, 2, 8, serve to bring the mirror into its correct position ; it is 
ressed against them Sy a band of india-rubber attached to the 
edges of the mirror. It will be seen that owing to this arrange- 
ment, all the different parts have motions quite oe 
and yet by the clamping screws can at any moment be connected. 
Finally, attached to this stand is a long light rod of sot iB 
reaching to the observer, and enabling him, by varying the 
_ tance between this movable piece and the fixed screen, to effect 
Compensation. 
Flexible Gas tubes.—It occurred to me that by splitting 
Stream of gas and sending one portion t to L, fig. 1, the other A 
two illuminations, as it would seem that any cause which in- 
creased the seamrlearye in one of the branches of the tube ought 
to be equally operative in the other, so that after a compensation 
had been effected it should be permanent. In practice 1 this was 
