18 
ANTHROPOLOGY: E. H. MORRIS 
Proc. N. A. S. 
The important results so far obtained from this work, which is still in 
progress, may be summarized as follows: 
1. Contrary to current belief, 1 there appears to be no such phenomenon 
as the direct reflection of an electron from a copper surface, or if the phe- 
nomenon exists at all, the number so reflected at potentials less than the 
ionizing potential is negligible. 
2. The result obtained in (1) reveals, therefore, a method of determining 
with simplicity and with considerable accuracy the ionizing potential of 
a metal surface. (The careful determination of the values of such po- 
tentials is now being carried out.) 
3. As the energy of electronic impact increases, the amount of re- 
emission from copper increases up to about 300 volts, from which point on 
it remains essentially constant. 
4. In the case of copper, the number of electrons detached per impinging 
electron is never found to exceed 1.3. 
5. The number of secondaries per electron is somewhat increased by 
cleaning the surface by heat, and it appears to be somewhat decreased 
when the temperature of the surface is raised. 
6. The maximum energy of emission of electrons released by electronic 
bombardment of copper increases from about 2 volts to about 5 volts 
as the energy of impact increases from 10 volts to 300 volts. 
The full details of this work will be reported by one of us in a more ex- 
tended article in the Physical Review. 
1 von Baeyer, O., Verh. deut. physik. Ges., Berlin, 10, 1908 (903); also Gehrts, A., 
Ann. Physik., Leipzig, 36, 1911 (996). Horton and Davies, Proc. Roy. Soc, 97, 23, 
1920, obtained results which they interpret in terms of reflections, but which we do 
not think demand such interpretation. The article by Hull, A. W., Proc. Inst. Radio 
Engineers, Feb., 1918, should also be read, though it does not bear upon this particular 
point. 
CHRONOLOGY OF THE SAN JUAN AREA 
By Karl H. Morris 
American Museum of Natural History, New York 
Communicated by H. F. Osborn, December 2, 1920 
The chronological outline herein presented is the result of ten years' 
research among prehistoric aboriginal remains in that portion of the San 
Juan watershed lying in northwestern New Mexico, southwestern Colorado, 
and adjacent areas in Arizona and Utah. In the earlier stages of the work, 
financial support was given successively by the School of American 
Archaeology, the St. Louis branch of the Archaeological Institute of 
America, and the University of Colorado. For the past five years the 
work has been under the direction of the American Museum of Natural 
History as a part of the Archer M. Huntington Survey of the Southwest. 
