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



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

 most troublesome with a very narrow slit. 



§ 223. 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 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 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 half way across the field so 

 that the two spectra shall have about the same width. 



§ 224. Scale of Wave Lengths. — In the Abbe micro-spec- 

 troscope 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 jx. 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 frequenty used and serves to illustrate § 213-214, is 

 to sprinkle some salt of sodium (carbonate of sodium is good) in a 

 Bunsen or alcohol lamp flame and to examine this flame. If this is 

 done in a darkened 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 



