58 MICRO-SPECTROSCOPE AND POLAR/SCOPE. 



ened place with a spectroscope, two narrow bright yellow bands will be 

 seen in the yellow part of the spectrum. If now ordinary daylight is sent 

 through the comparison prism, the bright lines of the sodium will be 

 seen to be directly continuous with the dark lines at D in the solar spec- 

 trum (PI. V, Fig. 45). 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 bands. 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 posi- 

 tion 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. 



§ 139. 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- 

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

 measurements (§ 108). 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 of one thousandth mm., or 0.5892 /x. The last 

 would be indicated thus, XD = 0.5892 fx. 



§ 140. 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 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 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 

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

 by experience. For many objects some good artificial light is better 

 than daylight. 



