CH. IV] MICRO-SPECTROSCOPE AND POLAR/SCOPE 139 



\ 196. Complementary Spectra. — While it is believed that Angstrom's law 

 (I 194) is correct, there are many bodies on which it cannot be tested, as they 

 change in chemical or molecular constitution before reaching a sufficiently high 

 temperature to become luminous. There are compounds, however, like those of 

 didymium, erbium and terbium, which do not change with the heat necessary to 

 render them luminous, and with them the incandescence and absorption spectra 

 are mutually complementary, the one presenting bright lines where the other 

 presents dark ones (Daniell). 



ADJUSTING THE MICRO-SPECTROSCOPE 



§ 197. The micro-spectroscope, or spectroscopic ocular, is put in 

 the place of the ordinary ocular in the microscope, and clamped to the 

 top of the tube by means of a screw for the purpose. 



§ 198. Adjustment of the Slit. — In place of the ordinary dia- 

 phragm with circular opening, the spectral ocular has a diaphragm 

 composed of two movable knife edges by which a slit-like opening of 

 greater or less width and length may be obtained at will by the use of 

 screws for the purpose. To adjust the slit, depress the lever holding 

 the prism-tube in position over the ocular, and swing the prism aside. 

 One can then look into the ocular. The lateral screw should be used 

 and the knife edges approached till the}' appear about half a millimeter 

 apart. If now the Amici prism is put back in place and the micro- 

 scope well lighted, one will see a spectrum by looking into the upper 

 end of the spectroscope. If the slit is too wide, the colors will overlap 

 in the middle of the spectrum and be pure only at the red and blue 

 ends ; and the Fraunhofer or other bands in the spectrum will be 

 faint or invisible. Dust on the edges of the slit gives the appearance 

 of longitudinal streaks on the spectrum. 



§ 199. Mutual Arrangement of Slit and Prism. — In order 

 that the spectrum may appear as if made up of colored bauds going 

 directly across the long axis of the spectrum, the slit must be parallel 

 with the refracting edge of the prism. If the slit and prism are not 

 thus mutually arranged, the colored bands will appear oblique, and 

 the whole spectrum may be greatly narrowed. If the colored bands 

 are oblique, grasp the prism tube and slowly rotate it to the right or to 

 the left until the various colored bands extend directly across the spec- 

 trum. 



§ 200. Focusing the Slit. — In order that the lines or bands in 

 the spectrum shall be sharply defined, the eye-lens of the ocular should 

 be accurately focused on the slit. The eye-lens is movable, and when 

 the prism is swung aside it is very easy to focus the slit as one focused 



