CH. VL] MrCRO-SPECTROSCOrE AND POLAR/SCOPE. 125 



ADJUSTING THE MICRO-SPECTROSCOPE. 



§ 188. 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. 



§ 189. Adjustment of the Slit. — In place of the ordinary diaphragm 

 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 approach 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 spectro- 

 scope. 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 Fraun- 

 hofer 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. 



§ 190. 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 mutu- 

 ally 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. 



§ 191. 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 (§ 161). If one now uses daylight there will be 

 seen in the spectrum the dark Fraunhofer lines (Fig. 116 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 dif- 

 ferent 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 



