Vol. 7, 1921 
PHYSICS: DUANE, PALMER AND YEH 
237 
The demonstration of a change in sequence of identical loci that is here 
reported makes the identification of parallel mutations in species that 
cannot be crossed even more difficult than it has previously seemed; for 
identity of sequence in a group of identical loci now appears not to be 
necessarily expected. 
0.0 
dachs 
0.0 
roughoid 
14.2 
scarlet 
39.0 
deltoid 
41.5 
scarlet 
44.5 
peach 
76.5 
peach 
63.5 
delta 
101.0 
minute-23 
FIGURE 1 FIGURE 2 
1 Contribution from the Carnegie Institution of Washington. 
2 Sturtevant, A. H., Genetics, 6, 1921 (63, 179). 
3 Discovered by Prof. T. H. Morgan. 
4 Discovered by Dr. C. B. Bridges. 
5 This map is based on the more extensive one published by Bridges in these Pro- 
ceedings. 
6 Bridges, C. B., J. Gen. Physiol, 1, 1919 (645). 
7 Weinstein, A., these Proceedings, 6, 1921 (625). 
8 It is, of course, possible to invert this interpretation by supposing the simulans 
situation to be the original one. 
9 Muller, H. J., Amer. Nat., 50, 1916 (103, 284, 350, 421), and Sturtevant, A. H., 
Carnegie Inst. Wash. PubL, No. 278, 1919 (305). 
10 Sturtevant, A. H., these Proceedings, 3, 1917 (555), and Carnegie Inst. Wash. 
PubL, No. 278, 1919 (305). 
A REMEASUREMENT OF THE RADIATION CONSTANT, h, BY 
MEANS OF X-RAYS 
By Wiixiam Duane, H. H. Palmar and Chi-Sun Y3h 
Jefferson Physical Laboratory, Harvard University 
Communicated July 6, 1921 
Since the discovery of the fact 1 that the continuous X-ray spectrum 
has a short wave-length limit, which obeys the quantum law, a number of 
experimentors have used this phenomenon to determine the value of h. 2 
In its application to X-rays the quantum law may be expressed by the 
equation 
Ve = hv, (1) 
where V represents the maximum difference of potential in the X-ray tube 
through which the electrons fall, e, the charge carried by each electron, 
v, the frequency of vibration corresponding to the short wave-length 
limit of the spectrum, and h, Planck's action constant. Evidently a 
measurement of V and v gives us the ratio of h to e, and from this we 
get h, if we suppose e to be given by other experiments. Blake and Duane 3 
