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



cordance with the general law that the waves of shortest length, blue, etc., 

 will be bent most, the colors have the position indicated in the top of Fig. 138, 

 also above Fig. 134. But if one looks into the direct vision spectroscope or 

 holds the eye close to the single prism (Fig. 139), the colors will appear re- 

 versed as if the red were more bent. The explanation of this is shown in Fig. 

 139, where it can be readily seen that if the eye is placed at E, close to the 

 prism, the different colored rays appear in the direction from which they 

 reach the eye and consequently are crossed in being projected into the field 

 of vision and the real position is inverted. The same is true in looking into 

 the micro-spectroscope. The actual position of the different colors may be 

 determined by placing some ground glass or some of the lens-paper near the 

 prism and observing with the eye at the distance of distinct vision.* 



VARIOUS KINDS OF SPECTRA 



By a spectrum is meant the colored bands appearing when the light 

 traverses a dispersing prism or a diffraction grating, or is affected in any way 

 to separate the different wave lengths of light into groups. When daylight 

 or some good artificial light is thus dispersed one gets the appearance so 

 familiar in the rainbow. 



\ 212. Continuous Spectrum. — In case a good artificial light as the elec- 

 tric light is used the various rainbow or spectral colors merge gradually into , 

 one another in passing from end to end of the spectrum. There are no breaks 

 or gaps. 



I 213. Line Spectrum. — If a gas is made incandescent, the spectrum it 

 ' produces consists, not of the various rainbow colors, but of sharp, narrow, 

 bright lines, the color depending on the substance. All the rest of the spec- 

 trum is dark. These line spectra are very strikingly shown by various metals 

 heated to incandescence. 



§ 214. Absorption Spectrum. — By this is meant a spectrum in which 

 there are dark lines or bands in the spectrum. The most striking and inter- 

 esting of the absorption spectra is the Solar Spectrum, or spectrum of sunlight. 

 If this is examined by a good spectroscope it will be found to be crossed by 

 dark lines, the appearance being as if one were to draw pen marks across a 

 continuous spectrum at various levels, sometimes apparently between the 

 colors and sometimes in the midst of a color. These dark lines are the so- 

 called Fraunhofer Lines. Some of the principal ones have been lettered with 

 Roman capitals. A, B, C, D, E, F, G, H, commencing at the red end. The 

 meaning of these lines was for a long time enigmatical, but it is now known 

 that they correspond with the bright lines of a line spectrum (§213). For 

 example, if sodium is put in the flame of a spirit or Bunsen lamp it will 

 vaporize and become luminous. If this light is examined there will be seen 

 one or two bright yellow bands corresponding in position with D of the solar 



*The author wishes to acknowledge the aid rendered by Professor E. L. 

 Nichols in giving the explanation offered in this section. 



