CH. Vr\ MICRO-SPECTROSCOPE AND POLAR/SCOPE 143 



justing 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 oppo- 

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

 serves to illustrate § 191-2, is to sprinkle some salt of sodium (carbon- 

 ate of sodium is good) in an 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 at D in the solar spectrum (Fig. 121). 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 /a) 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 war 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. 



§ 203. Designation of Wave Length. — Wave lengths of light 

 are designated by the Greek letter A., followed by the number indicat- 

 ing the wave length in some fraction of a meter. With the Abbe 

 micro-spectroscope the micron is taken as the unit as with other micro- 

 scopical measurements (§ 166). Various units are in use, as the one 

 hundred thousandth of a millimeter, millionths or ten millionths of a 

 millimeter. If these smaller units are taken, the wave lengths will be 

 indicated either as a decimal fraction of a millimeter or as whole num- 

 bers. Thus, according to Angstrom, the wave length of sodium light 

 is 5892 ten millionths mm., or 589.2 millionths, or 58.92 one hundred 

 thousandths, or 0.5S92 one thousandth mm., or 0.5892 /<. The last 

 would be indicated thus, A. D = 0.5892 /(. 



