256 DESIGNATING WAVE LENGTHS OF LIGHT [Cn. VIII 



(carbonate of sodium is good) in a Bunsen or alcohol flame and to 

 examine this flame. If this is done in a darkened place with a spectro- 

 scope, 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 con- 

 tinuous with the dark line at D in the solar spectrum (fig. 148). By 

 reflecting light into the scale-tube the image of the scale will appear 

 on the spectrum, and by a screw just under the scale-tube, but within 

 the prism- tube, the proper point on the scale (0.589^1) can be brought 

 opposite the sodium band. All the scale will then give the wave 

 lengths directly. Sometimes the scale is oblique to the spectrum. 

 This may be remedied by turning the prism-tube slightly one way or 

 the other. It may be due to the wrong position of the scale itself. 

 If so, grasp the milled ring at the distal end of the scale-tube and, 

 while looking into the spectroscope, rotate the tube until the lines 

 of the scale are parallel with the Fraunhofer lines. It is necessary 

 in adjusting the scale to be sure that the larger number, 0.70, is at 

 the red end of the spectrum. 



The numbers on the scale should be very clearly defined. If 

 they do not so appear, the scale-tube must be focused by grasping 

 the outer tube of the scale-tube and moving it toward or from the 

 prism-tube until the scale is distinct. In focusing the scale, grasp 

 the outer scale-tube with one hand and the prism-tube with the other, 

 and push or pull in opposite directions. In this way one will be less 

 liable to injure the spectroscope. 



406. Designation of wave length. Wave lengths of light are 

 designated by the greek letter X followed by the number indicating 

 the length in some fraction of a meter. See fig. 144 where the visible 

 spectrum is indicated as lying between wave lengths (X) 0.7/1 and 

 o.4/x. In this book the micron (/*) is taken as the unit as with other 

 minute measurements. Other units are also employed, especially 

 smaller ones so that the wave lengths will appear as whole numbers 

 instead of decimal fractions. The two other units having the greatest 

 sanction are the Angstrom unit (A.U.) and the millimicron (mju). 

 See 246. Expressed in Angstrom units the limits of the visible 

 spectrum would be: \jooo to X4ooo A.U.; and if in millimicrons 

 the limits are: X7oo to X4OO m/z. The wave length of sodium 



