188 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
Now if you permit these somewhat arbitrary assumptions to 
Bohr, he can and does deduce, at least for the lighter atoms such as 
hydrogen and helium, the Rydberg formula for the spectral series. 
He finds: 
= ie Ry 
where n is the frequency; m, e, mass and charge of an electron; h is 
Planck’s constant; a, 6, are integers. The quantity before the 
bracket should equal the Rydberg number N,, of observed value 
3.2910". Bohr’s calculated value is 3.2610, showing a most 
satisfactory agreement. 
Bohr endeavors to account for the manner in which two hydrogen 
atoms form a molecule. Each atom has a nucleus of positive charge 
and a simple electron revolving around it. Their charges are equal 
and opposite. The nuclei of two such atoms repel each other. The 
revolving electrons of two atoms close together, if rotating in the same 
direction, constitute two parallel currents of electricity, and these 
attract one another and arrive in the same plane. It is easy to make a 
model on a whirling table with the nuclei on an upright rod, the 
electrons revolving like the governor balls of an engine. Bohr has 
gone further, and conceived a similar model of a water molecule with 
the two nuclei of hydrogen and one nucleus of oxygen in a straight 
line, with 10 electrons revolving in their zones around them. No 
doubt these suggestive schemes are somewhat speculative, but it is 
refreshing to find a first approximation to a dynamical scheme re- 
placing the old unsatisfactory electrostatic atoms, which probably 
did not approximate to the truth. Some of the formidable organic 
molecules must have a complexity which it may take generations of 
physicists to unravel. 
11. One of the triumphs of mathematical physics was the forecast 
of Laue that crystal bodies have their atoms so distributed that 
Roéntgen rays must be diffracted by these atoms in the same manner 
that closely ruled cross lines diffract visible light. This forecast and 
its rapid verification, enable the two Braggs, father and son, to meas- 
ure with accuracy the wave lengths of Réntgen rays. While the 
waves of visible light are of the order 1075 centimeter, those of Rént- 
gen rays are of the order 107° centimeter, about one-thousandth of 
the former. The electromagnetic theory recognizes no intrinsic 
difference between the great waves of wireless telegraphy, several 
kilometers in length (10° centimeters), short electric waves, long heat 
waves, visible light (10-5 centimeter), ultra-violet waves, and Rént- 
gen rays (107° centimeter). 
The method of reflecting Réntgen rays from a rock-salt or another 
crystal has been applied by Moseley with marked success to the 
