To view Fraunhofer’s lines on the solar spectrum it is only necessary to turn the knife edges towards a white cloud, and make the slit formed by the knife edges very narrow by turning the screw at the side of them. In every instance the focus of the telescope must be adjusted in the ordinary way, by sliding the draw tube until it suits the observer’s sight, and distinct vision is obtained.
It should be noted that lines at various parts of the spectrum require a different adjustment in focussing the telescope.
The small prism turning on a joint in front of the knife edges is for the purpose of showing two spectra in the field of view at the same time. To do this it must be brought close to the front of the knife edges. Then one flame must be placed in the position in which the flame of the candle is shown in the small figure, and the other directly in front of the slit. On looking through the telescope, as before described, the spectra due to the two substances will be seen one above the other.
When the slit is turned towards a bright cloud, and a light is used in the position of the candle flame, the spectrum of any substance may be seen compared with the solar spectrum. In this manner Kirchhoff determined in the solar spectrum the presence of the lines of the greater number of the elements which are believed to exist in the sun.
PROFESSOR STOKES’ ABSORPTION BANDS.
This instrument is expressly adapted to the prismatic analysis of organic bodies, according to the method recommended by Professor Stokes in his lecture at the Chemical Society, printed in the Chemical News.
To observe these bands it is only necessary to place a very dilute solution of the substance in a test-tube, then fix the test-tube in the small clip attached to a ring which slips on in front of the knife edges. Upon bringing any bright light in front of the tube, on looking through the telescope, if the instrument has been properly adjusted, a bright spectrum will be seen, interrupted by the dark bands due to the substance in solution.
One of the simplest and most interesting experiments of this kind can be made by preparing dilute solutions of madder, port wine, and blood.
In these very dilute solutions no difference can be detected by the unassisted eye, but on submitting them in the manner already described to the test of spectrum analysis, very different appearances will be presented.
The absorption bands may, however, be most conveniently examined and accurately investigated by means of Sorby and Browning’s new Micro-Spectroscope.