Ch. VIII] ARRANGING THE SPECTROSCOPE 255 



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

 focus of the Unes at the blue end. This is because the wave length 

 of the red is markedly greater than for blue light (fig. 144). 



Longitudinal dark lines on the spectrum may be due to irregularity 

 of the sht or to the presence of dust. They are most troublesome 

 with a very narrow slit. 



§ 404. Comparison or double spectrum. — In order to compare 

 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 niicro-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 wiU be side by side, with 

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

 and dayhght be sent through the microscope and into the side to the 

 reflecting 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 

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

 about halfway across the field so that the two spectra shall have 

 about the same width. 



§ 405. Scale of wave lengths. — In the Abbe micro-spectroscope 

 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 

 previous 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 

 adjusting the scale, one may focus very sharply the dark sodium hne 

 of the solar spectrum and set the scale so that the number 0.589 is 

 opposite the sodium or D line, or a method that is frequently used 

 and serves to illustrate § 394-39S, is to sprinkle some salt of sodium 



