LYMAN. — TIIE SPECTRUM OF HYDROGEN. lol 
Six circular plates of white fiuorite 3 mm. thick and 2.5 cm. in diameter, and two 
plates 2 mm. thick — all from Zeiss of Jena — have been tested, with the result that, 
while none of them are absolutely opaque to light below \1G00, their transparency 
varies very much. In no case, however, was any line of wave-length shorter than 
X 1200 obtained, and of the eight pieces but two showed this transparency. Tl * 
abrupt nature of the absorption at this point is well shown by spectra II and III in 
Plate VIII. II was taken with the internal capillary discharge tube and fiuorite win- 
dow, III with no window between tube and slit. The author is not of course prepar. 1 
to say that no fiuorite does exist transparent to light below XI 200, he can only say 
that of the best specimens obtainable up to the present, but two show even this 
limited transparency. The discovery of some substance transparent to light of the 
very shortest wave-length known to exist would be an important Step. For our 
knowledge of the spectra of gases other than hydrogen is at present limited by the 
transparency of fiuorite. 
The effect of the thickness of the fiuorite window was tested by taking a series of 
spectrographs through one of the two best specimens and then reducing the thickness 
of the piece from 3 to 0.9 mm. A second series taken through this thinner window 
showed no extension of the spectrum whatsoever. This is a result which might have 
been expected from the work of Schumann and which confirms, for this region, that 
slow increase of absorption with thickness which has been observed in other parts of 
the spectrum. 
ABSORPTION OF THE AIR. 
The absorption of the air is the important factor in all investigations which have 
do with radiations of short wave-length. Cornu was the first 
n 
systematically, but Sch 
tly extended the work and has given 
data on the relation of length of air path to the limit of the spectrum. His method 
was to interpose a cell whose thickness could be varied between his source of light 
and the slit of his spectroscope. This cell was filled with air at atmospheric 
pressure. 
There is not much to add. The method here employed was as follows : The 
discharge tube was separated from the spectroscope by a fiuorite window and pectro- 
air in the receiver. Thus the light from the discharge tube 
graphs were taken with 
sed a layer of fiuorite and then passed through air to the grating and 
back 
the photographic plate -a distance of about 200 cm. By taking a series of spectro- 
graphs at different pressures the variation of the absorption with pressure could be 
