OSCILLATIOX-FREQUENCIES OF SOLAR RAYS. 39 



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wave-lengths of those rays of KirchhofFs list which are not found in Ang- 

 strom's, wherever it appeared O possiblc to make the interpolation with safety. 



The small corrections which Angstrom indicates at p. 29 of his memoir ('Le 

 Spectre Normal du Soleil ') have been applied to his numbers before insert- 

 ing them in column 2. Accordingly the numbers of this column which are 

 not in brackets represent Angstrom's work in its finished state. 



In column 6 the intensities and widths which Kirchhoff assigns to rays 

 between A and G have been reproduced ; and Mr. Burton has continued 



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these determinations to all the rays recorded by Angstrom between G and U, 

 so that they now cover the whole spectrum from A to H. Before entering on 

 this work Mr. Burton prepared himself by a revision of portions of the spec- 

 trum which Kirchhoff had delineated, so as to ensure that he should employ 

 KirchhofFs symbols in the same sense in which they had been used 1)y Kirch- 

 hoff and his assistant Hofmann. 



And in the last column Mr. Burton has thrown the solar rays into such 

 groups as appeared to him to be the most convenient to an observer. It will 

 probably be possible to improve this part of the work, if a second edition of the 

 Catalogue is called for. 

 • In columns 3 and 4 are given the steps by which the oscillation-frequencies 

 of the rays have been computed from their wave-lengths, in order that it may 

 be easy to revise the former if improvements are at any time made upon 



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Angstrom's table of wave-lengths, or on the values for the refraction of air 

 which have been used. 



The Map which the Committee arc engaged in preparing will bo a mere 

 chart, in which the intensities of the rays will be indicated by lines of diffe- 

 rent lengths. It does not appear to the Committee to be desirable that they 

 should attempt a finished drawing of the Solar Spectrum in the present state 

 of spectroscopic science, in which observers may hope soon to have in their 

 hands good photographs of every part of the visible spectrum. In order 

 meanwhile to supply as far as possible the place of a more finished map, 

 tables will be appended which will enable any one 'who possesses KirchhofTs 



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exquisite map or Angstrom's to place upon them the outlines of a scale of 

 oscillation-frequencies, so as to make these maps in a large degree available. 

 The Committee will feel obliged to any spectroscopists who are so good as 

 to send to G. Johnstone Stoney, 3 Palmerston Park, Dublin, such corrections of 

 the present tables as may occur to them, with a view to their insertion in 

 future editions. 



Catalogue of the principal Dark Bays of the visible part of the Solar Spec- 

 trum, containing all the rays registered by Kirchhoff and Angstrom, 

 arranged on a scale of Oscillation-frequencies. 



Explanation'. 



Column 1 gives the position on the Arbitrary Scale attached to Kirchhoffs maps. 



Column 2 reproduces the waye-lengths in tenth-metres as determined by Angstrom, after 

 applying to the numbers of Angstrom's list the small corrections which lie indicates 

 at p. 29 of his memoir, " Le Spectre Normal du Soleil." The wave-lengths of rays 

 recorded by Kirchhoff, but not by Angstrom, hare been introduced within brackets 



