S9S PKOCEEDINGS OF THE AMERICAN ACADEMY 



InvestjGations on* Light and Heat, made and published wholly or iu part with 

 appropriation from the Rumfokd Fuxd. 



XXII. 



ON THE GROUP ''6" IN THE SOLAR SPECTRUM. 



By William C. Wixlock. 



Presented by Prof. Wolcott Gibbs, June 9, 1880. 



The most complete charts of the solar spectrum now available are 



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Kirchhoff's, which were published in ]861, and Angstrom's, published 

 in 1869. KirchhofF employed a battery of four flint-glass prisms, with 

 a collimator and observijig telescope each of about 4 centim. aperture 



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and 49. centim. focal length ; while Angstrom used telescopes of about 

 4.6 centim. aperture, and 36.3 centim. focal length, and a diffraction 

 grating made by Nobert, containing about 133 lines to the millimetre. 

 Such great advances have been made very recently in the construc- 

 tion of optical instruments, and more especially in the ruling of 

 diffraction gratings, that it would now be possible to enlarge Angstrom's 

 great chart almost as much as he improved upon Frauiiliofer's first 

 maps. But it would be an almost endless undertaking for a single 

 observer to attempt a map of the whole spectrum, from the ultra- 

 violet to the invisible red, brought to light by our most powerful in- 

 struments, and accordingly most physicists who have paid especial 

 attention to solar spectroscopy have devoted themselves to a careful 

 study of detached portions which appear of unusual interest. As a 

 slight contribution to this work, the following observations upon the 

 group of dark lines " h" of the solar spectrum, were undertaken at the 

 suggestion of Dr. Gibbs, and carried on under his immediate super- 

 vision. 



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A table of wave-lengths, based upon the wave-lengths of Angstrom's 

 lines, has been computed, and, in addition to this, the wave-lengths of 

 all new lines within the limits adopted, have been deduced by graphi- 

 cal interpolation, in order to give a check upon the values obtained 

 for X in the table on page 403. 



The group "i" is readily distinguished about the middle of the 

 green by its two bold components visible with very low dispersive 



