Fraunhofer Lines of Diffraction and Prismatic Spectra. 143 



A beam of light is directed by the silvered plane mirror of a 

 heliostat (A) into a darkened room. 



It is received on an achromatic lens (B) 10 centims. in dia- 

 meter; focal distance from posterior surface 70 centims. 



A slit (C) is then placed within the focus of this lens, the 

 distance being 48 centims. from the lens (B). 



After passing through the narrow slit, which is about one tenth 

 of a millimetre wide, the light is received upon a second achro- 

 matic lens (D), of the same diameter as the first, but with a focal 



^ss:% 



distance of 115 centims. The distance of this lens from the slit 

 is 164 centims. ; and the focusing of the lines of the spectrum on 

 a paper screen or on the ground glass of the camera is accom- 

 plished by moving the lens (D) nearer to or further from the 

 slit (C), or by moving the camera or screen (F) itself. 



The grating (E), mounted on a suitable stand, is placed at a 

 distance of 80 centims. from the second lens. All parts of the 

 apparatus being carefully adjusted, so that A, B, C, D, E are 

 on the same horizontal axis, the grating is then arranged on its 

 vertical axis, to throw the centre of its reflected image on the 

 opening of the slit (C). 



The lines of the grating being accurately parallel to the sides 

 of the slit, a series of beautiful spectra are produced on each 

 side of the slit, any or all of which may be received on suitably 

 adjusted screens, one of which is represented at F. In all of 

 these spectra, if the slit is very narrow, the prominent Fraun- 

 hofer, with numerous other lines, appear sharply defined. 



Of the spectra described above, only the first, second, and 

 third orders on each side of the image of the slit are available 

 for general use, on account of the overlapping of those that fol- 

 low. Of those that are available, I have preferred to use the 

 second order, since in this the dispersion is much greater than 

 in the first, and by the apparatus described above a spectrum 

 of a length of more than 30 centims. is obtained. 



For the projection of the prismatic spectrum a prism is sub- 

 stituted in place of the grating, when a very fine spectrum is 

 produced, the focus of the violet end of which is very much closer 

 to the prism than that of the red end. 



In the diffraction -spectra, also, it is necessary to vary the angle 

 at which the screen is placed to define sharply the lines at the 



