CJ/. VI] MICRO-SPECTROSCOPE AND POLARISCOPE 165 



at D in the solar spectrum (Fig. 136). 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 (0.589/0 can be brought opposite the 

 sodium band. All the scale will then give the wave lenghts 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 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 gasping 

 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. 



225. 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 microspectroscope the micron is taken as the unit as with 

 other microscopical measurements ( 182). 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 tenth meters or Angstrom 

 units, or 5892 ten millionths mm., or 589.2 millionths, or 58.92 one 

 hundred thousandths, or 0.5892 one thousandth mm., or 0.5892^. 

 The last would be indicated thus, A 0=0.5892 /<. 



226 Lighting for the Micro-specftroscope. For opaque 

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

 mirror or condensing lens. For transparent objects the amount of 

 the substance 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 



