THE SPECTEA OF SOME OF THE FIXED STAES. 
417 
the last face of the prism nearest the small telescope. All the pencils therefore which 
emerge from the prism are, by the motion of the telescope, caused to fall nearly centri- 
cally upon its object-glass. -The micrometer screw has 50 threads to an inch; and 
each revolution is read to the hundredth part, by the divisions engraved upon the head. 
This gives a scale of about 1800 parts to the interval between the lines A and H of the 
solar spectrum. During the whole of the observations the same part of the screw has 
been used ; and the measures being relative, the inequalities, if any, in the thread of this 
part of the screw do not affect the accuracy of the results. The eye lens for reading the 
divisions of the micrometer-screw is shown at s. 
The mirror f receives the light to be compared with that of the star-spectrum, and 
reflects it upon the prism e, in front of the slit d. This light was usually obtained from 
the induction spark taken between electrodes of different metals, fragments or wires of 
which were held by a pair of small forceps attached to the insulating ebonite clamp 
r. Upon a moveable stand in the observatory was placed the induction coil, already 
described by one of us*, in the secondary circuit of which was inserted a Leyden jar, 
having 140 square inches of tinfoil upon each of its surfaces. The exciting battery, 
which, for the convenience of being always available, consisted of four cells of Smee’s 
construction, with plates 6 inches by 3, was placed without the observatory. Wires, in 
connexion with this and the coil, were so arranged that the observer could make and 
break contact at pleasure without removing his eye from the small telescope. This was 
the more important since, by tilting the mirror f, it is possible, within narrow limits, 
to alter the position of the spectrum of the metal relatively to that of the star. An 
arrangement is thus obtained which enables the observer to be assured of the perfect 
correspondence in relative position in the instrument of the stellar spectrum and the 
spectrum to be compared with it. 
5. The satisfactory performance of this apparatus is proved by the very considerable 
dispersion and admirably sharp definition of the known lines in the spectra of the sun 
and metallic vapours. When it is directed to the sun, the line D is sufficiently divided 
to permit the line within it, marked in Kirchhoff’s map as coincident with nickel, to 
be seen. The close groups of the metallic spectra are also well resolved. 
When this improved apparatus was directed to the stars, a large number of fine lines 
was observed, in addition to those that had been previously seen. In the spectra of all 
the brighter stars which we have examined, the dark lines appear to be as fine and as 
numerous as they are in the solar spectrum. The great breadth of the lines in the 
green and more refrangible parts of Sirius and some other stars, as seen in the less per- 
fect form of apparatus which was first employed, and which band-like appearance was 
so marked as specially to distinguish them, has, to a very great extent, disappeared ; 
and though these lines are still strong, they now appear, as compared with the strongest 
of the solar lines, by no means so abnormally broad as to require these stars to be placed 
in a class apart. No stars sufficiently bright to give a spectrum have been observed to 
* Philosophical Transactions, 1864, p. 141. 
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