142 MICRO-SPECTROSCOPE AND POLARISCOPE [CH. VI 



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 neces- 

 sary 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 sharpest 

 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 irregular- 

 ity of the edge of the slit or to the presence of dust. They are most 

 troublesome with a very narrow slit. 



§ 201. Comparison or Double Spectrum. — In order to com- 

 pare the spectra of two different substances it is desirable to be able to 

 examine their spectra side by side. This is provided for in the better 

 forms of micro-spectroscopes by a prism just below the slit, so placed 

 that the light entering it from a mirror at the side of the drum shall be 

 totally reflected in a vertical direction, and thus parallel with the rays 

 from the microscope. The two spectra will be side by side with a 

 narrow dark line separating them. If now the slit is well focused and 

 daylight be sent through the microscope and into the side to the reflect- 

 ing or comparison prism, the colored bands and the Fraunhofer dark 

 lines will appear directly continuous across the two spectra. The 

 prism for the comparison spectrum is movable and may be thrown en- 

 tirely out of the field if desired. When it is to be used, it is moved 

 about half way across the field so that the two spectra shall have 

 about the same width. 



S 202. Scale of Wave Lengths. — In the Abbe micro-spectro- 

 scope the scale is in a separate tube near the top of the prism and at 

 right angles to the prism-tube. A special mirror serves to light the 

 scale, which is projected upon the spectrum by a lens in the scale-tube. 

 This scale is of the Angstrom form, and the wave lengths of any part 

 of the spectrum may be read off directly, after the scale is once set in 

 the proper position, that is, when it is set so that any given wave 

 length on the scale is opposite the part of the spectrum known by pre- 

 vious investigation to have that particular wave length. The point 

 most often selected for setting the scale is opposite the sodium line 

 where the wave length is, according to Angstrom, 0.5892 /<. In ad- 



