THE 
AMERICAN JOURNAL OF SCIENCE 
[THIRD SERIES.] 
Oe 
ArT. XXXVII.—On the Wave Length of the D, Helium 
Inne; by A. DEForEsT PALMER, JR. 
Owine to the recent increased interest in the wave length 
of the helium lines due to the discovery of terrestrial helium, 
I have been led to calculate some observations on the D, chro- 
mosphere line carried out by myself at the Physical Laboratory 
of the Johns Hopkins University during February and March, 
1893. 
The measurements were made on the large fixed telescope 
spectrometer, used by Dr. Louis Bell* in his determination of 
the absolute wave length of the D solar lines, with a plane 
speculum metal grating having about fourteen thousand lines 
to the inch and five inches of grating space. The telescopes 
of this instrument are 16°4™ clear aperture and about 2°5 
meters focal length; and with the grating used I obtained 
good dispersion and excellent definition in the first spectrum 
to the right of the normal tothe grating. All the observations 
were made in this spectrum on account of its superior defini- 
tion. 
An image of the sun about 1™ in diameter was formed on 
the slit of the instrument by aid of a large Foucault heliostat, 
and an achromatic lens of about four inches aperture. Appli- 
ances were provided for moving the image laterally across the 
slit, and, by means of a total reflecting prism, for turning it 
about the direction of the beam as an axis to bring any desired 
point of the limb over the slit. | 
* This Journal, vol. xxxv, p. 265. 
Am. Jour. Sct.—Tarrp Series, Vou. L, No. 299.—NoOvEMBER, 1895. 
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