MR. W. GEOFFREY DUFFIELD ON THE 
112 
As this paper is the first record of work undertaken with the large Rowland Grating- 
in the Physical Laboratory of the University of Manchester, the occasion seems a 
fitting one for the description of the mounting of the instrument, which differs in 
several essential features from previously described systems.* 
A detailed account of the scheme adopted for the adjustment of the apparatus is 
also given in the hope that it will be of service to those who may be confronted with 
a similar task. The papers of Ames and KayserT on the adjustment of their own 
systems of mounting have been consulted and detailed acknowledgment will be found 
in the text. 
PART I. 
The Mounting of the Large Rowland Concave Grating. 
The grating possesses 14,438 lines to the inch; the length of the ruled portion is 
0 inches, its height 2 inches, and the radius of curvature of the concave metal surface 
upon which it is ruled 21 feet 6 inches. 
Rowland’s original system of mounting has, in general, been followed, though 
several important modifications have been introduced by Professor Schuster and by 
the constructor, Sir Howard Grubb. Two carriages, I)D, fig. 1, which run freely 
upon two heavy girders. A and B, placed at right angles to one another, carry the 
grating and camera and are connected by a cross-beam, C, which is of such a length 
that the centre of the curved photographic plate is at the centre of curvature of the 
mirror. The slit is placed at the point of intersection of the two girders, which are 
shown in plan in fig. 1. 
As the camera is moved away from the slit the grating moves towards it, the 
method of attachment being such that both turn with the cross-beam, which remains 
normal to the curved surfaces of the mirror and the photographic plate. With this 
disposition the spectrum should be in focus for all positions of the carriages along 
their girders. The mounting has been specially designed to make the various 
adjustments independent of one another and to ensure the stability of the carriages 
and their freedom from any constraint due to a possible twisting or sagging of the 
cross-beam. 
The Girder for the Grating Carriage. (Fig. 1, A.) 
This is a heavy cast-iron girder of LJ section, 20 feet 6 inches long, 6^ inches high, 
8 inches wide, whose upper edges have been machined, one to form a A and the other 
flat. These carry the wheels of the grating carriage. KayserI had previously 
* Ames, ‘Phil. Mag.’ (5), 27, 369, 1889. Kaysek, ‘Hanclbuch der Spectroscopie,’ i., p. 473. Baly, 
“Spectroscopy,” ‘Textbook of Physical Chemistry Series,’ p. 195. Adeney and Carson, ‘Proc. Roy. 
Soc. Dublin’ (1), 8, 711 (1898). 
t ‘ Handbuch der Spectroscopie,’ i., 473. 
