SOME COXTF.MPOK.IRV .IDr.lXCliS IX I'llVSICS IV Mf> 

 iiuMii> to .1 (li>t,iiit raiim> which ni.-\c-r can In- cxli'iulfd to oxcrlap llio 



OlhiT. 



The ailvaiHi- into the lacuna from the- iliroclion of sliortcr vvaveS, 

 iliat is, from llic s|uctrum of natural rays, came almost to a stop in 

 191 1, at a wavclcnijlli bolwccn ()..'i and 0.4 mm. Rubens and von 

 Hacycr cxamiiic<l the rays emitted l)y a mercury vapor arc in a (juartz 

 tulx', oi)crated with a comparatively high expenditure of power; 

 they filtered the radiation through a succession of diaphragms and 

 lenses which cut out a large fraction of the short-wave radiation, 

 but not by any means all of it. At first they analyzed the ratliation 

 which came through with an interferometer, like the one which I shall 

 descril)e in speaking of short artificial waves; the curves indicated 

 that it consisted largely of two waves, one at 0.218 mm. and the 

 other at 0.343 mm. RuIkmis in 1921 returned to the experiments, 

 and diffracted the transmitted rays with a large-scale wire grating 

 (the wires were a millimetre thick and a millimetre apart). This 

 method of analyzing the radiation, in which it is spread out in a 

 sfHictrum, is preferable to the other. The results were quite con- 

 cordant with the earlier ones; the curves of intensity versus wave- 

 length show maxima at 0.210 mm. and 0.325 mm., and extend out 

 as far as 0.4 mm.' There is no sign that this is a definite physical 

 limit; it is merely the point at which the rays become too feeble to 

 protluce an unmistakable deflection of the micro-radiometer. Nichols 

 and Tear also have observed these long natural waves. 



To advance into the lacuna from the region of artificial waves, it 



was found necessary first of all to remodel the oscillator or "doublet" 



by means of which Hertz had generated the first waves of this kind. 



The original oscillators of Hertz were rather large; some for example, 



consisted of pairs of metal plates 40 cm. square or pairs of spheres 



30 cm. in diameter with arms projecring from each toward the other 



and carrying knobs several mm. or cm. in diameter; their natural 



frequencies were of the order 10'— 10". Their successors were made 



[)rogressively smaller, and the latest oscillators are comparatively 



minute — in dealing with a less exact science, one would describe 



them as microscopic; for Mobius before the war used a doublet of 



' It is not necessarily to be assumed that the mercury arc is unique in sending 

 out rays of so great a wavelength with so great an intensity; these rays may not 

 he more intense than a black body of the same temiwrature as the arc would emit in 

 this portion of its spectrum Uilthough this interpretation would involve a rather 

 high estimate of the arc temperature, many thousands of degrees). But if we had a 

 black Ixxly of this temperature available, we might not be able to detect these rays be- 

 lause of the flood of light of higher frequencies which could not be completely deflected 

 from the path of the long waves. Thus we arc led to the para<loxical conclusion 

 that the mercury arc may be unique not in furnishing these rays, but in not emitting 

 so much radiation of lesser wavelengths that the rays desired cannot be isolateil. 



