OF SOME OF THE STAES AND NEBULAE. 
537 
consists of two compound prisms, d and e, and three simple prisms, f, g, h. Each of the 
compound prisms contains five prisms, cemented together with Canada balsam. The 
shaded portions of the diagram represent the position of the two prisms of very dense flint 
glass in each compound prism. The compound prism marked e is much larger than the 
other, and is permanently connected with the telescope c, with which it moves. These 
compound prisms, which were made specially to my order by Hofmann, are of great per- 
fection, and produced severally a dispersion fully equal to two prisms of ordinary dense 
flint glass. The prisms/* and g were cut for me from a very fine piece of dense glass of 
Guinand by Messrs. Simms, and have each a refracting angle of 60°. The prism h was 
made by Mr. Browning from the dense flint glass manufactured by Messrs. Chance, this 
prism has a refracting angle of 45°. The great excellence of all these prisms is shown 
by the very great sharpness of definition of the bright lines of the metals when the induc- 
tion-spark is taken before the slit, even when considerable magnifying power is employed 
on the small telescope with which the spectrum is viewed. The instrument is provided 
with a second collimator, of which the object-glass has a focal length of 18 inches. 
The compound prism e is so fixed that it can be removed at pleasure, when the total 
dispersive power of the instrument is reduced from about six and a half prisms of 60°, 
to about four and a half prisms of 60°. The facility of being able to reduce the power 
of the instrument has been found to be of much service for the observation of faint 
objects, and also on nights when the state of the atmosphere was not very favourable. 
The telescope with which the spectrum is viewed is carried by a micrometer-screw, 
which, however, has not been employed for taking measures of the spectra, but only for 
the purpose of setting the telescope to the part of the spectrum which it is intended to 
observe. This precaution is absolutely necessary when nebulae are observed which emit 
light of two or three ref'rangibilities only. 
For the purpose of the simultaneous comparisons of the light of the heavenly bodies 
with the lines of the terrestrial elements, the slit was provided, in the usual way, with a 
small prism placed over one half of it, which received the light reflected upon it from a 
small mirror placed opposite the electrodes. The plan of observation formerly employed, 
and which is described in the paper “ On the Spectra of some of the Fixed Stars,” was 
adopted to ensure perfect accuracy of relative position in the instrument between the 
star-spectrum and the spectrum to be compared with it, since it is possible by tilting the 
mirror to alter within narrow limits the position of the spectrum of the terrestrial sub- 
stance relatively to that of the star. Before commencing an observation, a small alcohol- 
lamp, in the wick of which bicarbonate of soda was placed, was fixed before the object- 
glass of the telescope, and then the mirror and the electrodes were so adjusted that the 
components of the double line D were exactly coincident in both spectra. 
This plan was soon found to be very inconvenient, and even in some degree untrust- 
worthy for the more delicate comparisons which were now attempted. An unobserved 
accidental displacement of the spark, or of the mirror, might cause the two spectra to 
differ in position by an amount equal to the whole extent of want of coincidence which 
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