VISUAL, GRATING AND GLASS-LENS, SOLAR SPECTRUM. 527 



The first step in reduction consisted in transferring with ruling pen and 

 square all these fiducial markings at their several distances apart, to large 

 engraved sheets, furnished with scales of nearly equal length to that of the 

 Micrometric apparatus, but with much more space above and below whereon to 

 develop the condensed symbology of the original pencillings, and introduce 

 dates and notes for every day's work. Each of the three spectra was thus 

 treated in perfect independence of the others, and the third one had further still 

 been observed in a very different manner to the first and second. For while 

 they had the benefit of a collimator 70 inches long, the collimator used for the 

 third, though with the same diameter of objective, was only 35 inches long; 

 while the cone of rays from the preliminary condensing lens of the heliostat w r as 

 now still further condensed and shortened by a supplementary lens. 



Greater brightness of spectrum was hoped for by this concentration of Sun- 

 light on the slit, but was not obtained ; and in place of it only a third spectrum 

 very like the first and second ; — no one of them being perfect ; and the mean of 

 the three, probably better than any one taken by itself alone, 



The amounts of such residual failings in each set of measures, though very 

 small in themselves and hardly to be perceived in most spectrum work, was 

 nevertheless, on the grand scale here attempted, quite sufficient to prevent the 

 records, though derived from a Diffraction Grating, being always read off 

 implicitly as a continuous scale of wave-lengths. I determined therefore to look 

 upon them as varying differential measures, to be trusted only for short micro- 

 metric runs ; while an absolute scale was prepared for them all, by referring the 

 places of their chief lines to Angstrom's celebrated Normal Solar Spectrum 

 Map, — assisted where necessary by the numbers recorded in the much later 

 works of MM. Vogel, Cornu, Fievez, Thollon, and Professor Young, U.S. Am. 



Their scales however being for Wave-lengths in terms of the French 

 Metre, — I had to reduce their figures to Wave-number per British Inch, for 

 the reasons stated in my paper lately printed by the Royal Society, Edinburgh, 

 entitled " Micrometrical measures of Gaseous Spectra." While finally I 

 entered the leading divisions of such a scale, to the number of 600 for each 

 spectrum, upon the sheets of pen and ink work, — after having obtained the 

 rates of variation by the usual method of projecting the places of known lines 

 on paper ruled transversely with the Micrometer seale, and drawing curves, 

 through the points of intersection. The insertion of nine marks of nearly equal 

 subdivision between every pair of the original 600 then followed, and gave each 

 spectrum by itself a Wave-number scale with as many as 6000 fiducial steps 

 marked and numbered ; but on a continually contracting, or conical, scale in 

 passing from Red to Violet : as well as totally different when, for some parts of 

 the spectrum, — as the very earliest of the Red, — the first order of the Grating 

 was substituted for the second order, or that usually observed upon. 



VOL. XXXII. PART III. 4 S 



