Fig. 191.—Object-glass Prism.
In the other arrangement we have the thing managed in a different way: we have the object-glass collecting the light from the star and bringing it to a focus on the slit, and it then passes on to the prisms, through which the light has to pass before it comes to the eye. In this combination of telescope and spectroscope we have what has been called the telespectroscope; one method of combination is seen in the accompanying drawing of the spectroscope attached to Mr. Newall’s great refractor; but any method will do which unites rigidity with lightness and allows the whole instrument to be rotated with smoothness.
Fig. 192.—The Eyepiece End of the Newall Refractor (of 25 inches aperture), with Spectroscope attached.
For solar observation, as there is light enough to admit of great dispersion, many prisms are employed, as shown in Fig. [192]; or the prisms may be made so tall that the light may be sent backwards and forwards many times by means of return prisms, to which reference has been already made.
For the observation of those bodies which give a small amount of light, fewer prisms must be used, and arrangements are made for the employment of reference spectra, i.e., to throw the light coming from different chemical elements into the spectroscope, in order that we may test the lines; whether any line of Sirius, for instance, is due to the vapour of magnesium, as Kirchhoff tested whether any line in the sunlight was referable to iron or the other vapours which he subsequently studied.
Fig. 193.—Solar Spectroscope (Browning’s form).
