Vol.. 8, 1922 
ASTRONOMY: W. W. COBLENTZ 
49 
equivalent to a given thickness of copper. The ratio of the equivalent 
thicknesses of aluminium and copper changes a great deal in this 
short wave-length region of the X-ray spectrum. In the neighborhood 
of wave-length .3, for instance, an aluminium plate must be twenty-eight 
times as thick as a copper plate in order to absorb the same fraction of 
X-radiation. For wave-length .095, however, a plate of aluminium 
only 6 times as thick as a plate of copper will absorb the same fraction of 
X-radiation. The estimate of the equivalent thickness of aluminium and 
copper can be made comparatively easily and rapidly, so that the method 
furnishes a good means of determining the "effective" wave-length of a 
beam of X-rays during a treatment with them. 
The curve giving the relation between the equivalent thickness and the 
wave-length will be published in a technical journal. The equivalent 
thickness for a given short wave-length can be calculated, of course, from 
equations (3), by dividing the linear coefficient of absorption for copper 
by that for aluminium. 
The coefficient of absorption of a beam of rays that is not monochro- 
matic usually (but not always) decreases as the rays pass through matter. 
If one measures the coefficient of absorption of such a beam, the results 
give a kind of average coefficient of absorption for the rays, as they travel 
through the absorbing material. Similarly, the above-mentioned meth- 
ods of measuring the "effective" wave-lengths furnish a kind of average 
value for the "effective" wave-length of the beam as the rays pass through 
the plates. 
1 Ithaca, Physic. Rev., August 1915, p. 166. 
2 lUd., Sept. 1916, p. 326. 
^ Ibid., July 1921, p. 13. 
THE EFFECTIVE TEMPERATURE OF i6 STARS AS ESTIMATED 
FROM THE ENERGY DISTRIBUTION IN THE COMPLETE 
SPECTRUM 
By W. W. CoBLKNTz 
Bureau of Standards, Washington, D. C. 
Communicated by V. M. Slipher, January 23, 1922 
1. Introductory Statement, — Data on the spectral energy distribution 
of stars as related to that of a black body are very meager. They are the 
results practically of the spectrophotometric measurements of Wilsing,^ 
and of Nordmann,^ and the spectral energy curves determined photo- 
graphically by Plaskett,^ all of which relate to the visible spectrum. 
The various methods used to obtain stellar temperatures give different 
results. For example, Plaskett (loc. cit.) obtained an effective tem- 
perature of 6800° K for y Cassiopeiae (Class Bop) by considering the con- 
tinuous spectrum and 10,600° K by considering the bright line spectrum. 
