OF SOME OF THE STAES AND NEBULAS. 
539 
to meet the requirements of the case. In place of the small prism, two pieces of sil- 
vered glass were securely fixed before the slit at an angle of 45°. In a direction at right 
angles to that of the slit, an opening of about inch was left between the pieces of 
glass for the passage of the pencils from the object-glass. By means of this arrange- 
ment the spectrum of a star is seen accompanied by two spectra of comparison, one 
appearing above, and the other below it. As the reflecting surfaces are about 0'5 inch 
from the slit, and the rays from the spark are divergent, the light reflected from the 
pieces of glass will have encroached upon the pencils from the object-glass by the time 
they reach the slit, and the upper and lower spectra of comparison will appear to over- 
lap to a small extent the spectrum formed by the light from the object-glass. This 
condition of things is of great assistance to the eye in forming a judgment as to the 
absolute coincidence or otherwise of lines. For the purpose of avoiding some incon- 
veniences which would arise from glass of the ordinary thickness, pieces of the thin 
glass used for the covers of microscopic objects were carefully selected, and these were 
silvered by floating them upon the surface of a silvering solution. In order to ensure 
that the induction-spark should always preserve the same position relatively to the 
mirror, a piece of sheet gutta percha was fixed above the silvered glass ; in the plate of 
gutta percha, at the proper place, a small hole was made of about inch in diameter. 
The ebonite clamp containing the electrodes is so fixed as to permit the point of sepa- 
ration of these to be adjusted exactly over the small hole in the gutta percha. The 
adjustment of the parts of the apparatus was made by closing the end of the adap ting- 
tube, by which the apparatus is attached to the telescope, with a diaphragm with a 
small central hole, before which a spirit-lamp was placed. When the lines from the 
induction-spark, in the two spectra of comparison, were seen to overlap exactly, for a 
short distance, the lines of sodium from the light of the lamp, the adjustment was con- 
sidered perfect. The accuracy of adjustment has been confirmed by the exact coincidence 
of the three lines of magnesium with the component lines of b in the spectrum of the moon. 
In some cases the spectra produced by the spark are inconveniently bright for com- 
parison with those of the stars and nebulse. If the spark is reduced in power below a 
certain point, many of the lines are not then well developed ; the plan, therefore, was 
adopted of diminishing the brightness of the spectrum, by a wedge of neutral-tint glass 
which can be moved at pleasure between the plate of gutta percha and the silvered 
mirror. 
Two eyepieces were employed with the apparatus, the one magnifying four diameters, 
and the other six diameters. 
The induction-coil was the same which I employed in my former observations. It 
was excited by a form of bichromate-of-potash battery, which I have found so exceed- 
ingly convenient for the occasional work of celestial observations that I will describe it 
here. 
The battery, which was made for me by Mr. Ladd, consists of two large cells of 
ebonite, each of which contains two plates of graphite, 6 inches by 7^ inches, connected 
