II.] 



FRAUNHOFER'S OBSERVATIONS. 



15 



cannot be over-estimated, will be gathered from the following 

 extract from his communication to the Munich Academy : x 



" Into a dark room, and through a vertical aperture in the 

 window-shutter, about 15" broad and 36" high, I introduced the 

 rays of the sun upon a prism of flint glass placed upon the 

 theodolite ; this instrument was 24 feet from the window, and the 

 angle of the prism was nearly 60 3 . The prism was placed before 

 the object glass of the telescope, so that the angles of incidence and 

 emergence were equal. In looking at this spectrum for the bright 



FIG. 7. Fraunhofer's theodolite spectroscope. 



line which I had found in the spectrum of artificial light, I dis- 

 covered, instead of this line, an infinite number of vertical lines of 

 different thicknesses. These lines are darker than the rest of the 

 spectrum, and some of them appear entirely black. When the 

 prism was turned so that the angle of incidence increased, these 

 lines disappeared, and the same thing happened when the angle was 

 diminished. If the telescope was considerably shortened, these lines 

 reappeared at a greater angle of incidence ; and at a smaller angle 



1 Denkschriften de K. Acad. der Wixscnschaften zu Munchen, 1814-15, Band v. 

 pp. -193 -226. Translated in Edinburgh Philosophical Journal, vol. ix. p. 296, 

 1823, and vol. x. p. 26, 1824. 



