134 MICRO-SPECTROSCOPE AND POLAR/SCOPE. [CH. VI. 



tion Spectra. — Some quite brilliantly colored objects, like the skin of 

 a red apple, do not give a banded spectrum. Take the skin of a red 

 apple, mount it on a slide, put on a cover-lass and add a drop of water 

 at the edge of the cover. Put the preparation under the microscope 

 and observe the spectrum. Although no bands will" appear, in some 

 cases at least, yet the ends of the spectrum will be restricted and vari- 

 ous regions of the spectrum will not be so bright as the comparison 

 spectrum. Here the red color arises from the mixture of the unab- 

 sorbed wave lengths, as occurs with other colored objects. In this 

 case, however, not all the light of a given wave length is absorbed, 

 consequently there are no clearly defined dark bands, the light is simply 

 less brilliant in certain regions and the red rays so predominate that 

 they give the prevailing color. 



§ 206. Nearly Colorless Bodies with Clearly Marked Absorp- 

 tion Spectra. — In contradistinction to the brightly colored objects with 

 no distinct absorption bands are those nearly colorless bodies and solu- 

 tions which give as sharply defined absorption bands as could be de- 

 sired. The best examples of this are afforded by solutions of the rare 

 earths, didymium, etc. These in solutions that give hardly a trace of 

 color to the eye give absorption bands that almost rival the Fraunhofer 

 lines in sharpness. 



§ 207. Absorption Spectra of Minerals.— As example take some 

 mouazite sand on a slide and either mount it in balsam (see Ch. VII), 

 or cover and add a drop of water. The examination may be made also 

 with the dry sand, but it is less satisfactory. Light well with trans- 

 mitted light, and move the preparation slowly around. Absorption 

 bands will appear occasionally. Swing the prism-tube off the ocular, 

 open the slit and focus the sand. Get the image of one or more grains 

 directly in the slit, then narrow and shorten the slit so that no light 

 can reach the spectroscope that has not traversed the grain of sand. 

 The spectrum will be very satisfactory under such conditions. It is 

 frequently of great service in determining the character of unknown 

 mineral sands to compare their spectra with known- minerals. If the 

 absorption bands are identical, it is strong evidence in favor of the 

 identity of the minerals. For proper lighting see § 195. 



§ 208. While the study of absorption spectra gives one a great deal 

 of accurate information, great caution must be exercised in drawing 

 conclusions as to the identity or even the close relationship of bodies 

 giving approximately the same absorption spectra. The rule followed 

 by the best workers is to have a known body as control and to treat the 

 unknown body and the known body with the same reagents, and to 



