ME. J. N. LOCKYEB ON SPECTEOSCOPIC OBSEEVATIONS OP THE SUN. 443 
in the case of F the bright line observed was sometimes on one side of the absorption- 
line, and sometimes put it out altogether (October 27). 
Figs. 7, 8, and 9 are outlines of the prominences observed on the 20th of October, 
5th and 6th of November 1868. 
Added October 10, 1869. 
PLATE XXXVIII. 
The spectroscope of large dispersive power, by means of which the work I have de- 
scribed in the former part of this paper has been done, is attached to a clock-work 
driven refractor of 6^ inches aperture and 98^ inches focal length, the definition of 
which is very fine, and worthy of the reputation of its makers, Messrs. Cooke and Sons, 
York. 
Fig. 1 will give a general idea of the spectroscope and of its attachment to the 
large equatoreal. In this figure are shown the eye-end of the telescope with finder 
and clamping and adjusting rods, and an internal tube which I have found it convenient 
to insert into the ordinary one to allow of the adjustment of the spectroscope to the 
direct solar image, and the variously enlarged images, without disturbing the spectro- 
scope itself on its supports. 
This internal tube carries the ordinary eyepiece-end on which the prismatic steel rods 
which support the train of prisms, collimator, and telescope are fixed and provided with 
means of rotation. 
Fig. 1 also shows the arrangement of the instrument when the direct image is viewed. 
The eyepiece is taken out, and the image is allowed to fall on the slit-screen outside 
the telescope. A short collimator collects the light which then passes through the 
seven prisms (of 45°) of dense flint glass, of which the specific gravity, refractive index, 
and dispersive power, as determined by Mr. Browning, are as follows : — 
Specific gravity 3*91 
Refractive index 1*665 
Dispersive power 0*0752 
Ordinarily the seven prisms are alone used, but when more dispersion is wanted I 
have found it very convenient to fix an extra prism of 60°, as shown in the figure; this 
makes the instrument for some part of the spectrum a direct vision one ; and I have 
further increased the dispersion by partly filling the small telescope with direct vision 
prisms. 
The adjustment of the spectroscope to the telescope allows of the slit being brought 
either tangentially or radially on any part of the sun’s limb. 
Fig. 2 shows a slit I have designed for comparing various portions of the solar surface 
with each other. The slit, for purposes of description, may be imagined to be divided 
into three portions. The central portion admits direct light from some part of the sun ; 
