254 ARRANGING THE SPECTROSCOPE [Ch. VIII 



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 microscope 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. 



§ 402. 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 

 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 spectrum. 



§ 403. 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 for the ocular micrometer (§ 240). If one now uses 

 daylight there will be seen in the spectrum the dark Fraunhofer lines 

 (fig. 146, 148). 



To show the necessity of focusing the slit, move the eye-lens down 

 or up as far as possible, and the Fraunhofer lines cannot be seen. 

 While looking into the spectroscope move the ocular lens up or down, 

 and when it is focused the Fraunhofer lines will reappear. As the 

 different colors of the spectrum have different wave lengths, it is 

 necessary to focus the slit for each color if the sharpest possible 

 pictures are desired. 



It will be found that the eye-lens of the ocular must be farther from 



