128 MICRO-SPECTROSCOPE AND POLAR/SCOPE. [CH. VI. 



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 on the spectrum may be due to irregularity 

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

 troublesome with a very narrow slit. 



§ 192. Comparison or Double Spectrum. — In order to compare 

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

 ine 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 reflecting or com- 

 parison 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 b: thrown entirely 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 spectrums shall have about the same 

 width. 



§ 193. 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 lines where the wave 

 length is, according to Angstrom, 0.5892 /*. In adjusting the scale, one 

 may focus very sharply the dark sodium line 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 § 171, is to 

 sprinkle some salt of sodium (carbonate of sodium is good) in an alco- 

 hol lamp flame and to examine this flame. If this is done in a dark- 

 ened place with a spectroscope, a narrow bright band will be seen 

 in the yellow part of the spectrum. If now ordinary daylight is 

 sent through the comparison prism, the bright line of the sodium 

 will be seen to be directly continuous with the dark line at D in the 



