56 



MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



end will be slightly, and the blue end considerably restricted, and some of the 

 colors will appear of considerably lessened intensity. Finally there may appear 

 two shadow-like bands, or if the layer is thick enough, two well-defined dark 

 bands in the green ($ 146). 



If the thick layer is examined in the same way, the spectrum will show only 

 red with a little orange light, all the rest being absorbed. Thus the spectroscope 

 shows which colors remain, in part or wholly, and it is the mixture of this remain- 

 ing or unabsorbed light that gives color to the object. 



\ 132. Complementary Spectra. — While it is believed that Angstrom's law (? 130) 

 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 MICROSPECTROSCOPE. 



§ 133. The micro-spectroscope or spectroscopic ocular is put in the 

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

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



§ 134. 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 they 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 over- 

 lap 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. 



§ 135. Mutual arrangement of Slit and Prism. — In order that 

 the spectrum may appear as if made up of colored bands 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 mu- 

 tually 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 spectrum. 



§ 136. 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 



