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Fan. 23, 1873 | 
persion, the spectrum is observed by the telescope on the 
left, which is simply a small astronomical telescope of 
low magnifying power. There are two methods of mea- 
suring spectra. The telescope may be attached to a 
NATURE 
227 
moveable arm, which can be directed to any part of the 
spectrum that may be required ; and the outer edge of the 
circle along which the telescope moves may be graduated 
with an accurate scale of degrees, which can be divided 
GLaecanre | 
Fic._15.—Spectroscope with reflected scale. 
with more or less minuteness, according to the’precision in 
the exact position of the dark lines, &c., in various spectra 
required. In this method the line to be measured is 
brought into the centre of the field of view of the observ- 
ing telescope, and the position of the telescope read off. 
Of course if the line measured is situated in the red end 
of the spectrum, the telescope will be in a different position 
to that it will occupy if the line be in the blue end. The 
Fic. 16.—Huggins’ star spectroscope, 
second method of measurement may be gathered from 
Fig. 15. It consists of a short tube carrying at its outer 
extremity a small photographic scale, which is illumi- 
nated by a candle flame; the light passing from the 
photographic scale is rendered parallel and thrown on 
the surface of the prism by means of a lens in the tube 
carrying the scale, and is reflected by the last surface 
of the prism up the observing telescope, so that it is seen 
as a bright scale on the background, formed by the 
| spectrum under observation. 
| The spectroscope has also been adapted to the tele- 
scope with very great success ; for it is essential not only 
to determine the spectra of the light emitted by various 
substances in our laboratories on this earth, but also the 
different spectra and positions of the dark lines or bright 
| ones, &c., obtained from the various orders of celestial 
s 
