142 REPORT—1868. 
produce that particular group of dark lines. In this way Kirchhoff disco- 
vered the presence of several terrestrial elements in the solar atmosphere. 
Mernops or OBSERVATION. 
The speaker then described the special methods of observation by which, 
in their investigations, they applied these principles of spectrum analysis to 
the light of the heavenly bodies. He stated that several circumstances unite 
to make these observations very difficult and very irksome. In our climate, 
on few nights only, even of those in which the stars shine brilliantly to the 
naked eye, is the air sufficiently steady for these extremely delicate observa- 
tions. Further, the light of the stars is feeble. This difficulty has been met, 
in some measure, by the employment of a large telescope. The light of a star 
falling upon the surface of its object-glass of eight inches aperture is gathered 
up and concentrated at the focus into a minute and brilliant point of light. 
Another inconyenience arises from the apparent movement of the stars, 
caused by the rotation of the earth, which carries the astronomer and his 
instruments with it. This movement was counteracted by a movement given 
by clockwork to the telescope in the opposite direction. In practice, how- 
ever, it is not easy to retain the image of a cine for any length of time 
exactly within the jaws of a slit only the =4, inch apart. By patient 
perseverance these difficulties have been overcome, and satisfactory results 
obtained. They considered that the trustworthiness of their results must 
rest chiefly upon direct and simultaneous comparisons of terrestrial spectra 
with those of celestial objects. For this purpose the apparatus which is re- 
presented in fig. 1, and fig. 2, Plate ILI., was contrived. 
By the outer tube ¢ the instrument is adapted to the eye-end of the tele- 
scope, and is carried round with it by the clock motion. Within this outer 
tube a second tube 6 slides carrying a cylindrical lens, a, This lens is for the 
purpose of elongating the round point-like image of the star into a short line 
of light, which is made to fall exactly within the jaws of a narrow slit, d. 
Behind the slit, an achromatic lens, g (and at the distance of its own focal 
length), causes the pencils to emerge parallel. They then pass into two 
prisms, h, of dense flint glass. The spectrum which results from the decom- 
position of the light by the prisms is viewed through a small achromatic 
telescope, 7. This telescope is provided with a micrometer screw, g, by which 
the lines of the spectra may be measured. 
The light of the terrestrial substances which are to be compared with the 
stellar spectra is admitted into the instrument in the following manner :— 
Over one half of the slit is fixed a small prism, e, which receives the light 
reflected into it by the moveable mirror f placed above the tube. The mirror 
faces a clamp of ebonite, 7, provided with forceps to contain fragments of the 
metals employed. These metals are rendered luminous in the state of gas 
by the intense heat of the sparks from a powerful induction coil. The light 
from the spark reflected into the instrument by means of the mirror and the 
little prism passes on to the prisms in company with that from the star. In 
the small telescope the two spectra are viewed in juxtaposition, so that the 
coincidence and relative positions of the bright lines in the spectrum of the 
spark with dark lines in the spectrum of star can be accurately determined. 
Moon anv PLANeTs. 
The speaker referred in a few words only to the spectra of the moon and 
planets. These objects, unlike the stars and nebule, are not original sources 
