CH. VII MICRO-SPECTROSCOPE AND POLARISCOPE 161 



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. 



§ 221. 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 paral- 

 lel 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. 



§ 222. 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 (§ 172). If one now uses, 

 daylight there will be seen in the spectrum the dark Fraunhofer 

 lines (Fig. 136 E. F., etc.). 



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 the slit for the sharpest focus of the red end than for the sharp- 

 est focus of the lines at the blue end. This is because the wave 

 length of red is markedly greater than for blue light. 



Longitudinal dark lines of the spectrum may be due to irregu- 



