382 W. LeConte Stevens— Recent Progress in Optics. 
argon, by Lord Rayleigh and Professor Ramsay ;* its remarka- 
ble property of green fluorescence when the electric spark is 
haa through it in presence of benzene, by Berthelot and 
eslandres ;+ and its association in meteoric iron and various 
minerals with helium, now proved to be a terrestrial as well as 
solar element, by Ramsay,{ Crookes, Lockyer, and others. 
With the diffraction spectroscope Rydberg,§ and Kayser and 
Rungel have discovered interesting relations among the spec- 
tral lines of a large number of terrestrial elements, arranging 
them into series whose distribution manifests chemical relation- 
sma quite analogous to that indicated in Mendeléeff’s periodic 
aw. 
By photographing the spectrum of Saturn’s rings and noting 
the relative displacement of the different parts of a spectral 
line, Keeler has obtained a beautiful direct proof of the 
meteoric constitution of these rings, a confirmation of the 
hypothesis put forth by Maxwell in 1859, that the outer por- 
tion of the rings must revolve more slowly than the inner por- 
tion, and yet not satisfy the conditions of fluidity. His work 
has been repeated and confirmed by Campbell** at the Lick 
Observatory. 
The spectro-heliograph devised by Halett has enabled him to 
photograph, on any bright day, not only the solar photo- 
sphere and spots but also the chromosphere and protuberances. 
He has made some remarkable attempts with this instrament 
to photograph the corona without an eclipse, unsuccessful thus 
far but not without promise of future success. 
Polarized Light. 
In the domain of polarized light, there have been several 
noteworthy recent researches. Nichols and Snowtt} have shown 
that calcite, though readily transparent for the brighter rays of 
the spectrum, rapidly diminishes in power of transmission for 
waves of short period, so that for the extreme violet this 
power is scarcely half so great as for the yellow. The trans- 
missive power of this crystal for the infra-red rays, between 
the wave length limits of 1 micron and 5°5 microns, has been 
investigated with the bolometer by Merritt,$§ who reaches the 
* Proc. Royal Society, Jan. 31, 1895. 
+ Comptes Rendus, June 24, 1895. 
Nature, April 4, May 16, July 4 and July 25, 1895. 
Wiedemann’s Annalen, 1893-1894. 
|| Ibid., 1888-1895. 
§| Astrophysical Journal, May, 1895, p. 416. 
** Tbid., August, 1895, p. 127. 
++ Astronomy and Astrophysics, March, 1893, p. 256. 
tt Philosophical Magazine, V, xxxili, p. 379. 
S§ Physical Review, May-June, 1895, p. 424. 
