USE OF SPECTROSCOPE IN ASTRONOMICAL OBSERVATION. 149 
form of instrument by which the solar spectrum is analysed. 
Fig. 10 represents the instrument by which Messrs. Kirchhoff 
and Bunsen carried out the original researches which resulted 
in the invention of spectroscopic analysis. The light of the sun 
is caused (by means of a heliostat) to fall upon an adjustable 
slit, shown at the farther end of the telescope A. The analysing 
power of the instrument is derived from the four prisms shown 
in the figure, and b is the telescope by which the spectrum is 
examined. A more powerful instrument on a similar general plan, 
but with several notable improvements and no less than nine 
prisms, has been constructed by Mr. Browning for M. Grassiot. 
A subject which recently attracted much attention points to a 
mode of applying the spectroscope which could hardly fail to 
yield valuable results in the hands of a capable and diligent 
observer. Mr. Huggins’ observation of the spectrum of the 
variable star T. Coronse, which suddenly blazed out in 1866, 
showed that light, which by all other methods of observation 
would not be separately perceptible, might be rendered cognisable 
by the spectroscope, for the continuous spectrum of the star 
was seen to be crossed by bright lines belonging to incandescent 
hydrogen. The whole light of hydrogen being collected into 
three fine lines, while that of the star was dispersed over a long 
spectrum, the former stood out, so to speak, upon the darker 
background of the continuous spectrum. It occurred to Mr. 
Lockyer that the property thus exhibited might avail to render 
the spectrum of the solar prominences perceptible, if these 
objects are gaseous. As all our readers are aware, the sugges- 
tion has been justified by the observations of Jannsens and 
Lockyer, and a promising means of spectroscopically examining 
the solar prominences and the neighbouring atmospheric enve- 
lope is thus afforded to astronomers. No special apparatus is 
requisite for the purpose. What is wanted is, that the image of 
a part of the sun’s limb should be brought across the slit at 
right angles to the slit’s length. Then, if the dispersive power 
of the spectroscope is sufficient, all that is necessary is to sweep 
the eye-telescope from end to end along the prism (a process 
which may require more than one adjustment, however); the 
spectrum of the prominence, if any occupy that part of the 
sun’s limb, will be indicated by the appearance of its character- 
istic bright lines opposite particular parts of the solar spectrum 
formed by the light from the limb. It need hardly be said that 
the telescope must be very accurately driven by the clockwork, 
and that no spectroscope which has not considerable dispersive 
power can be useful in work of this sort. 
