96 
PHYSICS: C. BARUS 
Proc. N. a. S. 
of view, we must suppose that the direction in which the characteristic 
ray travels depends upon neighboring atoms as well as upon the angle of 
incidence of the primary beam. 
The relative reflecting power of different planes may be determined by 
reflecting from them the same portion of the continuous spectrum, or differ- 
ent portions having known relative intensities. The use of the con- 
tinuous spectrum for this purpose instead of the line spectrum has the 
advantage of flexibility. One can employ short, penetrating X-rays, and 
with these investigate dense crystals of high absorbing power. 
1 Phys. Rev., Ithaca, Aug. 1915, p. 166. 
2 /. Optical Soc, May 1921, p. 386. 
ACOUSTIC TOPOGRAPHY VARYING WITH THE POSITION OF 
THE ORGAN PIPE"" 
By Cari^ Barus 
Department of Physics, Brown University 
Communicated March 20, 1922 
1. Introductory. — The coordinate axes have the x-y plane on the large 
table, the origin near one corner. The survey is made between walls 
at 7 = 174 cm. and y = —130 cm.; and the pipe is displaced both in 
the y and z (upward) direction. In the graphs, the letters P, R, T, T' , 
used either as subscripts or directly, will refer to the pipe, the reso- 
nator, the edges of the table. Throughout 5 is the fringe deflection of 
the U-tube interferometer, proportional to the acoustic pressure or to 
the nodal density evoked at a given place {% = 0, y, z), by the sounding 
pipe P. 
The latter was a closed brass f" pipe, blown by the pipe blower else- 
where described, rigidly attached. In graphs 1-3, this is sketched 
in under P and an arrow shows the direction of the wind current. The 
closed cylindrical pinhole resonator R, lying with its axis parallel to y, 
in the yx plane (table) and moving end-on along y, is also suggestively 
indicated, showing the resonator azimuth for which the usually paired 
graphs apply. In the earlier work (figures 1-3) the middle of the resona- 
tor was used for the location of points in y. An inversion of the reso- 
nator thus gave the two curves for each case and they were obtained con- 
secutively. If the mouth of R is used to determine the coordinate y 
as in figures 4, 6 to 10, but a single curve is obtained, for aU azimuths of 
R. The same is true for a resonator placed with its mouth at the mean 
yji of a given ordinate, s, in case of figures 1-3. 
2. Pipe Elevated above the Origin on the Table. — In figures 1-4 the or- 
gan pipe, with its mouth vertically above the origin, was raised succes- 
