120 
PHYSICS: C. BARUS 
tion of a general principle. That is, I am not certain. whether the accident of 
Nomadic invasion produced the outstanding Pueblo traits or whether in the 
absence of such pressure we should have observed the same phenomenon in all 
its essentials. In short, does or does not the 'age and area' hypothesis, com- 
monly subscribed to by students of organic life, hold true also in the province 
of human culture? But, passing over that detail, one thing seems fairly es- 
tabhshed both here in the Southwest and in several of our eastern culture areas. 
It is that North America north of Mexico, before it became settled by seden- 
tary agricultural tribes who developed many of the traits common to that 
type of life the world over, was settled by a generally more primitive nomadic 
type of peoples subsisting mainly by hunting, such as still persist over all of 
the northern and northwestern portions of the continent. The above sum- 
mary account is based upon data from the Archer M. Huntington Survey of 
Southwestern United States conducted by the American Museum of Natural 
History. The full report upon this survey will be published by the Museum. 
AN ADJUSTMENT IN RELA TION TO THE FRESNEL COEFFICIENT^ 
By Carl Barus 
Department of Physics, Brown University 
Communicated, February 10, 1919 
1. Apparatus. One internal reflection. — The specific part of the apparatus 
is the glass cylinder, G, figure 1, with a carefully polished mantel, capable of 
rotating around an axis, A , normal to the ray-plane of the interferometer. 
If micrometer facilities are to be dispensed with, and that is permissible in 
the present experiment,^ the interferometer may be designed as in figure 1. 
The white light L from the collimator takes the respective paths dCC'd'h and 
hd'C'Cd, the plate N being half silvered and N' an opaque mirror. The tele- 
scope or spectro-telescope is at T. The glass face at N may be turned either 
way. 
Such an interferometer is self adjusting (cf. preceding paper). In the form, 
figure 1, two reversed spectra will be visible in the telesbope, which if super- 
imposed by rotating TV or N' on a vertical axis, will show the linear phenom- 
enon at once, in any color at pleasure. The fringes may be enlarged by rotat- 
ing N or N' on a horizontal axis and they are symmetrically equal in size on 
the two sides of the adjustment for infinitely large fringes. 
If the achromatics are wanted, a prism must be inserted into the rays h 
(preferably) between N and N' , with a prism angle and other conditions 
selected to counteract the refraction of the cylinder G. 
2. Apparatus. Two internal reflections. — As the fringes were found with- 
out much difficulty (§5) in case of one internal reflection, it seemed desirable 
