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). ]
FIG. 194.—Solar Spectroscope (Grubb’s form). ]
FIG. 195.—Side view of Spectroscope, showing the arrangement by which the light from a spark is thrown into the instrument by means of the reflecting prism, _e_, by a mirror F. (Huggins.) ]
FIG. 196.—Plan of Spectroscope. T, eyepiece end of telescope, B interior tube, carrying A, cylindrical lens; D, slit of spectroscope; G, collimating lens; _h h_, prisms; Q, micrometer. (Huggins.) ]
FIG. 197.—Cambridge Star Spectroscope Elevation. ]
These are shown in Fig. 195. _e_ is a reflecting prism, and F is another movable reflector to reflect the light from a spark passed between two wires of the metal to be compared, and to throw it on the prism, which reflects the light through the slit of the spectroscope to the prisms and eye; if the instrument were in perfect adjustment and turned on a star, and a person were to place his eye to the spectroscope, he would see in one-half of the field of view the spectrum of the star with dark lines, and in the other half the spectrum of the vapour with its bright lines; and if he found the bright lines of the vapour to correspond with any particular dark line of the spectrum of the star, he would know whether the metal exists at that star or not; so this little mechanical arrangement at once tells him what there is at the star, whether it be iron or anything else.
In Figs. 197 and 198 is shown another form of stellar spectroscope, that of the Cambridge (U.S.) observatory; it is the same in principle as that just described.
A direct vision star spectroscope is shown in Fig. 199.
FIG. 199.—Direct-vision Star Spectroscope. (Secchi.) ]
A new optical contrivance altogether has to be used when star spectra are observed.