CH. VI ] MICRO-SPECTROSCOPE AND POLARISCOPE. 129 



solar spectrum (Fig. 1 14). Now, 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 in the prism-tube, the proper point on the 

 scale (0.589/*.) 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 

 slightl}' 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 neces- 

 sary 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 oppo- 

 site directions. In this way one will be less liable to injure the spec- 

 troscope. 



S 194. Designation of Wave Length. — Wave lengths of light are 

 designated by the Greek letter A., followed by the number indicating the 

 wave length in some fraction of a meter. With the Abbe micro-spec- 

 troscope the micron is taken as the unit as with other microscopical 

 measurements (§ 157). 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 numbers. 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 0.5892 one thousandth mm., or 0.5892 jt.. The last would be indi- 

 cated thus, A D = 0.5892 fi.. 



§ 195. Lighting for the Micro-spectroscope. — For opaque objects 

 a strong light should be thrown on them either with a concave mirror 

 or a condensing lens. For transparent objects the amount of the sub- 

 stance and the depth of color must be considered. As a general rule it 

 is well to use plenty of light, as that from an Abbe ' illuminator with a 

 large opening in the diaphragm, or with the diaphragm entirely removed. 

 For very small objects and thin layers of liquids it may be better to use 

 less light. One must try both methods in a given case, and learn by 

 experience. 



The direct and the comparison spectrums should be about equally 



9 



