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VI. — The Solar Spectrum at Medium and Low Altitudes. Observations oj the Region 

 between Wave-Lengths 6024 and 4861 A.U., made at Lord Crawford's Observa- 

 tory, Dun Edit, during the Years 1887 to 1889. By Ludwig Becker, Ph.D., 

 Temporary Second Assistant- Astronomer, Royal Observatory, Edinburgh. 



(Read 21st July 1890.) 



1. Introduction. 



At the end of 1886 a method occurred to me of rapidly recording the positions of the 

 lines of the solar spectrum, which I thought might be used with advantage for deter- 

 mining the faint " Telluric dry-gas lines" near D, mentioned in Professor Piazzi Smyth's 

 maps of The Visual Solar Spectrum in 1884-.* The fundamental idea of the recording 

 apparatus is that of magnifying by some mechanical means the motion of the grating, or 

 prism, to such an extent that it can be recorded on a continuous fillet of paper. The 

 viewing telescope being then firmly clamped, the exact positions of the grating can be 

 pricked off on the strip of paper as the lines are successively brought to the fixed cross 

 in the field of view. 



For the satisfactory use of the method two conditions suggest themselves as desirable. 

 First, the punctures should not be less than a good-sized pin hole, and the interval 

 between the closest lines, which the spectroscope is able to separate, should be represented 

 on the paper strip by a space of several tenths of an inch. Now, in one of Rowland's 

 gratings, the angular interval between the two positions of the grating which bring the 

 components of the E t line to the same direction is but 1 second of arc in the second 

 spectrum. To represent this small quantity by several tenths of an inch, we must either 

 use an enormous radius or multiply the angular movement some thousands of times. 



By way of experiment, I geared together on a board five pairs of well-finished wheels 

 and pinions, belonging to an excellent screw-cutting lathe, and observed under the highest 

 magnifying power of a microscope the motion of the slowest wheel, when the fastest one 

 was steadily turned by hand. The results being very satisfactory, Professor Copeland, 

 then Astronomer at Lord Crawford's Observatory, suggested continuing these experiments 

 with the head of the lathe mentioned. Soon afterwards Lord Crawford, whose atten- 

 tion had been drawn to the promising state of the problem, kindly decided to erect a 

 temporary station where the sun could be observed near the horizon. The place selected 

 was the top of the Barmekin, a commanding eminence about a mile and a half west of 

 Dun Echt Observatory. Lord Crawford also sent from his own workshop a horizontal 



* Trans. Roy. Soc. Edin., vol. xxxii. part 3. 

 VOL. XXXVI. PART I. (NO. 6). S 



