[ 223 ] 



XVII. On tho. Mounting of the Large Rowland Spectrometer 

 in the Rot/al University of Ireland. By W. E. Adeney, 

 D.Se.y F.I.C., Curator in the Royal University, and James 

 Carson, A.R.C.Sc.L, C.E.* 



THE working parts of this instrument were obtained from 

 Mr. J. A. Brashear, of Allegheny, United States of 

 America. They consist of two interchangeable J_ steel rails, 

 each of about 23 feet in length, and each having the upper 

 edged planed to an inverted A section with top truncated. 



These rails were supplied with saddles of cast iron, into 

 which the rail was fixed, and which were provided with 

 levelling and lateral adjustment screws. 



One of these saddles tikes one end of each rail at right 

 angles one to the other, and also carries the mounting for the 

 slit. 



The " diagonal beam " is an iron girder consisting of a tube 

 about 3 in. diameter (T,fig. 1) , trussed with |- in. rods (R, fig. 1), 

 the struts being placed at angles of 120° round the tube. 

 Fixed to each end of the tube is a cast-iron palm (J, fig. 1), 

 which is provided with a small range of adjustment in the 

 direction of the length of the girder. 



These palms are provided with vertical axes, which form the 

 connexion with the carriages (I, fig. 1), which run on the rails. 



On the one palm is fixed the grating-holder (G, fig. 1), and 

 on the other the camera. These parts are similar in structure 

 to those described and illustrated by Ames in the 'Astro- 

 Physical Journal,' p. 28, January 1892. 



The concave grating, which was also obtained through 

 Mr. Brashear, has a focal length of 21*5 feet ; the ruled 

 space is about 6 inches long, bearing 14,438 lines to the inch. 

 The spectra on one side of the grating are all bright ; the 

 first order on the other side is somewhat brighter than the 

 others. Mr. Brashear remarks, in a letter to one of us, that 

 " Professor Rowland states all the lines are clear and sharp," 

 and adds, " You are very fortunate in getting this grating, 

 for no one knows when we will get another." 



Our own experience with the grating fully corroborates 

 these remarks. The definition of the spectral lines afforded 

 by it is remarkably fine ; and we feel it due to Mr. Brashear 

 to express here the thanks, which have been conveyed to him 

 by letter, for the trouble and care he has so courteously taken 

 to furnish this University with such a very fine instrument. 



* From the ' Scientific Proceeding's of the Rc^al Dublin Society,' 

 vol. viii. (N. S.) pt. vi. pp. 711-716. Communicated by Prof. G. F. 

 FitzGerald, F.R.S. 



R2 



