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XV. Short Reports from the Chemical Laboratory of Trinity Col- 

 lege, Dublin. By J. Emerson-Reynolds, M.I)., M.R.I. A., 



Professor of Chemistry, University of Dublin*. 



No. 4. — On a New Form of Measuring-Apparatus for a 

 Laboratory-Spectroscope ]. 



THE measuring-apparatus for a laboratory-spectroscope 

 which I have been asked to describe, was fitted about a 

 year ago to an instrument in common use in the College La- 

 boratory, and has afforded very satisfactory results. My chief 

 aim in planning the arrangement was, to facilitate the measure- 

 ment and identification of spectral lines and the mapping of 

 spectra under circumstances admitting of little general illu- 

 mination. 



The spectroscope to which the apparatus is fitted has two 

 fixed flint-glass prisms, the refracting angle of each being 60°. 

 This instrument is shown in the annexed engraving. When 

 in use the prisms are covered by a brass cap provided with 

 openings for the collimating- and observing-telescopes. The 

 movable arm D that supports the observing-telescope also 

 carries a vernier which is moved with the telescope over a 

 graduated arc; and in this usual way the relative positions of 

 the several lines of a given spectrum can be determined. The 

 angular distance traversed in passing from the extreme red 

 to extreme violet is necessarily small, owing to the low dis- 

 persive power of the instrument ; but this, I need scarcely say, 

 is an advantage rather than the reverse in a spectroscope 

 which is commonly employed as an aid in ordinary qualitative 

 analysis. 



The graduations of the arc are unavoidably close and diffi- 

 cult to read in a feeble light ; consequently the eyes of the 

 observer become speedily tired and unfitted for the examination 

 of faint spectra. Nevertheless measurements made with the 

 graduated arc and vernier are, in my experience, more trust- 

 worthy and satisfactory than those obtained with even the best 

 photographed scale that I have had the opportunity of working 

 with. Desiring, then, to retain the method of direct angular 

 measurement, I sought to multiply the motion in such a manner 

 as to obtain wide readings on a convenient scale. After many 

 trials in different directions, the form of apparatus which I 

 shall now describe was finally adopted. 



* From the Scientific Proceedings of the Royal Dublin Sociesy ; com- 

 municated by the Author. 



t For Report No. 1, " On Glucinum : its Atomic Weight and Specific 

 Heat," see Phil. Mag. [V.J vol. iii. p. 38 ; for No. 2, On a New Mineral 

 Borate, and for No. o, On an Analysis of Lievvite by Mr. Early's Me- 

 thod, see Phil. Mag. [V.] vol. iii. pp.~ 284 and 287. 



