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So simple a matter as the narrowing of the aperture through 

 which the sunlight streamed before it fell upon the prism which 

 separates it into its constituent colors, would have sufficed to 

 show that the spectrum was crossed by dark lines, named, after 

 their discoverer, the Fraunhofer lines of the solar spectrum. 

 These may be readily enough observed, with no other appli- 

 ances than a slit in a shutter which is observed through an ordi- 

 nary prism of glass. Fraunhofer increased the power of the 

 combination enormously by observing with a telescope, and this 

 simple combination, omitting minor details, constitutes that 

 wonder of modern science, the spectroscope. As the power of 

 a telescope is measured by the closeness of double stars which 

 it can "resolve," so that of the spectroscope may be estimated 

 by the closeness of the spectral lines which it can separate. In 

 order to form an idea of the advance in the power of spec- 

 troscopes let us for a moment consider the map of the solar 

 spectrum. (Fig. i) * * (For Fig. i see colored plate in a 

 good text-book of physics or an encyclopedia. Ed.) 



The portion which is visible to the unaided eye extends from 

 the Fraunhofer line A to H; but by photography it may be 

 traced far into the ultra-violet region and by bolometric meas- 

 urements it is found to extend enormously farther in the region 

 beyond the red. In the yellow we observe a dark line, mark 

 D, which coincides in position with the bright light emitted by 

 sodium — as when salt is placed in an alcohol flame. It may be 

 readily shown by a prism of very moderate power that this line 

 is double, and as the power of the instrument increases, the dis- 

 tance apart, or separation, of this doublet furnishes a very con- 

 venient measure of its separating or revolving power. Of 

 course this separation may be effected by simple magnification, 

 but this would in itself be of no service, as the "lines" them- 

 selves would be broadened by the magnification in the same 



