72 
PHYSICS: C. BARUS 
Proc. N. A. S. 
block, BB', preferably of iron. The connection m must also be large in 
section, so as to admit of rapid flow from A to A '. The U-tube is charged 
with mercury MmM', M and M' being as shallow as possible to counter- 
act the tendency to vibration. Thin plane parallel glass plates, gg', 
round discs of equal thickness and diameter, are floated on the mercury, 
which act as mirrors for the interferometer beams, L' and L" ', and also 
materially check the tendency of the pool of mercury to vibrate. It 
would be desirable to be able to use the mercury surfaces at M and M' 
directly without the intervention of the plate; but within the city limits 
the fringes are unsteady and hard to find. 
Se 
The top of the iron block BB' is recessed as shown, to receive the plane 
parallel glass plates GG'. These like gg' must be equally thick; otherwise 
the fringes will be multiplied and faint. The annular space cccc between 
G and B is filled with resinous cement, poured in in the molten state. The 
air space A A' shut off in this way communicates with the atmosphere by 
two tubulures, t and t' , in the front side. 
The ray parallelogram of the quadratic interferometer of which L'L" 
are the interfering rays should be vertical. The displacements of the 
achromatic fringes of white light are read off by a telescope with an ocular 
micrometer (scale part 0.01 cm.). The fringes parallel to the divisions of 
the micrometer are conveniently made a scale part in size. The block 
BB' should be mounted separately from the interferometer. If it is 
placed on the base of the latter, all manipulations there shake the mercury 
in BB' and it is necessary to wait for subsidence. This, however, occurs 
very soon, so that the separate mounting is not absolutely necessary. 
Without manual interference the fringes are about as quiet as in a solid 
apparatus. 
2. Experiments. — To test this apparatus the air space A A' was left 
with a plenum of air. With A' communicating with the atmosphere, A 
was joined through t and a Alimentary capillary glass or metal tube, to 
an apparatus by which slight pressure could be applied. In the first 
trials I attempted to use a water manometer controlled by a micrometer 
screw; but the vibrations of the meniscus were at once impressed on MM' 
so that the fringes were hard to keep at rest. I then devised the apparatu s 
shown in figure 2, which is merely an adaptation of the pin valve of an 
oxygen tank, with a good micrometer screw, s, and stuffing box, n. The 
