SOMR CONTEMPOR.IRY Ain'ANCES IN PUYSICS-IIl 275 



a hrainh-tube ofx-ninj; near tlie slit into tlif latter chanilier. and so 

 inaint.iin in it a o>nsi(leral>ly lower density of gas than is required in 

 the discharge. Hopfield has succeeded in maintaining an atnirts- 

 phere of one kind of gas in the discharge-tube, and an atmosphere of 

 another and a more transparent kind of gas in the chamber; the two 

 gases are pre\ented from mingling i»y the same pumping-arrangement. 

 As for the measurement of wa\e-lengths from 12()()A down to about 

 1IK)A, it must be made with a concave diffraction-grating, which 

 separates rays of different wave-lengths and itself focusses them at 

 different places; for the rays cannot penetrate the prism of a prism 

 spectograph, or the lens which is commonly used' to focus the beams 

 diffracted by a plane grating. Rowland of Johns Hopkins, the first 

 great master of the art of making diffraction-gratings, ruled them both 

 upon plane and upon concave surfaces. The plane grating was so 

 much the more easily ruled, that the conca\e grating fell into desuetude; 

 but it l)ecanie invaluable as soon as Lyman began to work in the region 

 where the lenses extinguish the light. One might have anticipated that 

 it would refuse to diffract rays the wave-lengths of which are only one- 

 twentieth, one-fiftieth, even one one-hundredth of the spacing between 

 its lines; but as Lyman and Millikan advanced farther and farther 

 beyond the earlier limit of the ultra-violet, the concave grating proved 

 itself competent to an extent which would probably have astonished 

 its inventor. In one of Millikan's articles we may read an account of 

 the ruling of new gratings by Pearson of Chicago; the spacing of the 

 lines was by no means unusually small (about .500 per mm.) but they 

 were ruled "with a very light touch so as to leave a portion of the 

 original surface functioning in the production of spectra" — partly 

 so that successive rulings might be nearly alike, but chiefly because 

 if just half the original surface could be left intact, a large proportion 

 t)f the total radiant energy would be diffracted into the first-order 

 spectrum (this is the only usable one, because the higher-order images 

 forme<l by the small wave-length rays encroach on the first-order 

 images of the rays of greater wave-lengths). The arrangement of 

 apparatus in experiments with the concave grating has varied little 

 from the form which Lyman originally gave it. In Fig. 2 (from an 

 article by McLennan) one sees the cross-section of a large tubular 

 air-tight chamber, containing the grating at L (it is mounted on 

 a carriage Q sliding on rails O, P), the slit at 5 and the photographic 



' There is no apparent reason against using concave mirrors instead of lenses, 

 unless the multiple reflections consume too much of the light. Luckiesh mentions 

 an instrument designed with focussing mirrors of nickel (Houston, Proc. Roy. Soc. 

 Edinb., 1912), which, however, were found inferior to quartz lenses in the range 

 in which it was tested. 



