302 W. Crookes—Spectrum of Helium. 
bring them together. The card-board screen is shown (tipped 
over) in the drawing of the epicycloidal caustic. 
WUE, WI, WWI VUAN BWA VIM MMU A HOA VHA Vite 
Reflection from a Parabolic Surface. 
The axis of the reflecting surface must of course point 
directly at the sun, or rather a little below it. If the paper is 
fairly sensitive the exposure need not be over a minute or two, 
and the motion of the sun will give no trouble. If a longer 
exposure is necessary the mirror may be turned a trifle from 
time to time. 
These phenomena are adapted to projection, by employing a 
nearly parallel beam thrown along the screen, either directly 
or by means of a mirror. The reflecting surface and the slits 
should be much larger than the ones used for photographic 
representation. 
Berlin, Germany. 
Art. XX XIII.—The Spectrum of Helium* ; by 
WILLIAM Crookss, F.R.S. k 
In the Chemical News for March 29th last (vol. lxxi, p. 151) 
I published the results of measurements of the wave-lengths 
of the more prominent lines seen in the spectrum of the gas 
from cléveite, now identified with helium. The gas had been 
given to me by the discoverer, Professor Ramsay; and being 
from the first batch prepared, it contained other gases as 
impurities, such as nitrogen and aqueous vapor, both of which 
gave spectra interfering with the purity of the true helium 
spectrum. I have since, thanks to the kindness of Professors 
Ramsay and J. Norman Lockyer, had an opportunity of exam- 
ining samples of helium from different minerals and of con- 
* From an advanced proof sent by the author, 
