THE SPECTEA OF SOME OF THE FIXED STAES. 
415 
for 1863*. About the same time figures of a few stellar spectra were also published by 
SECCHif. In March 1863, the Astronomer Royal presented a diagram to the Royal 
Astronomical Society, in which are shown the positions of a few lines in sixteen stars 
Since the date at which our note was sent to the Royal Society our apparatus has 
been much improved, and in its present form of construction it fulfils satisfactorily 
several of the conditions required. 
§ II. Description of the Apparatus and Methods of Observation employed. 
4. This specially constructed spectrum apparatus is attached to the eye end of a 
refracting telescope of 8 inches aperture and 10 feet focal length, which is mounted 
equatorially in the observatory of Mr. Huggins at Upper Tulse Hill. The object- 
glass is a very fine one, by Alvan Clark of Cambridge, Massachusetts ; the equatorial 
mounting is by Cooke of York ; and the telescope is carried very smoothly by a clock 
motion. 
As the linear spectrum of the point of light which a star forms at the focus of the 
object-glass is too narrow for the observation of the dark lines, it becomes necessary to 
spread out the image of the star ; and to prevent loss of light, it is of importance that this 
enlargement should be in one direction only ; so that the whole of the light received by 
the object-glass should be concentrated into a fine line of light as narrow as possible, 
and having a length not greater than will correspond to the breadth of the spectrum 
(when viewed in the apparatus) just sufficient to enable the eye to distinguish with ease 
the dark lines by which it maybe crossed. No arrangement tried by us has been found 
more suitable to effect this enlargement in one direction than a cylindrical lens, which 
was first employed for this purpose by Fraunhofer. In the apparatus by which the 
spectra described in our “Note” of February 1863 were observed, the cylindrical lens 
employed was plano-convex, of 0*5 inch focal length. This was placed within the focus 
of the object-glass and immediately in front of the slit of the collimator. 
The present form of the apparatus is represented in Plate X. figs. 1 & 2, where the 
cylindrical lens is marked a. This is plano-convex, an inch square, and of about 14 
inches focal length. The lens is mounted in an inner tube, b, sliding within the tube c, 
by which the apparatus is adapted to the eye end of the telescope. The axial direction 
of the cylindrical surface is placed at right angles to the slit d , and the distance of the 
lens from the slit within the converging pencils from the object-glass is such as to give 
exactly the necessary breadth to the spectrum. 
In consequence of the object-glass being over-corrected, the red and, especially, the 
violet pencils are less spread out than the pencils of intermediate refrangibility ; so that 
the spectrum, instead of having a uniform breadth, becomes slightly narrower at the red 
end, and tapers off in a greater degree towards the more refrangible extremity §. 
* Vol. xxxv. p. 71. f Astronomische Nachrichten, No. 1405, March 3, 1863. 
t Monthly Notices, Eoy. Astron. Soc. vol. xxiii. p. 190. 
§ The experiment was made of so placing the cylindrical lens that the axial direction of its convex cylin- 
MDCCCLXIV. 3 K 
