tlie Large Spectrometer in the Royal University of Ireland. 225 



Two red deal beams, each 12 in. by 3 in., and of sufficient 

 length to carry the steel rails, were mounted true and level 

 on cast-iron standards bolted to the floor. 



To these were bolted the saddles mentioned above, and 

 on these were fixed the rails. The latter were very care- 

 fully adjusted, so as to be exactly level, straight, and at right 

 angles to one another. 



The beam on which the grating-rail H was mounted also 

 served as a basis for a light-tight wooden structure running 

 along its whole length, and completely enclosing both the 

 rail and grating. Fig. 1 shows this structure in section, with 

 the rail, carriage, and grating indicated in position inside. 



One end of this rectangular structure or box was perma- 

 nently closed, and a short piece of brass tube made to slide 

 through a hole in the end at the proper height for the slit. 

 This brass tube was supplied with a boxwood flange fitting- 

 light-tight against the slit mounting, so that all the adjust- 

 ments of the latter were outside the wooden rectangular box. 



The other end was closed by a sliding door. 



A light-tight connexion had now to be established between 

 this wooden box and the camera at the other end of the girder ; 

 and inasmuch as the girder not only moved from one end of 

 the tube to the other along the rail H, but at every new posi- 

 tion on the rail made a different angle with it, it was impos- 

 sible to accomplish this connexion by anything in the nature 

 of a bellows. After much consideration the following method 

 was decided on, and works admirably in practice : —On the 

 iron tube of the girder was fixed, by means of wooden sup- 

 ports and clamps, a wedge-shaped rectangular tube of wood, a 

 little wider than the grating at one end, and the width of 

 the camera at the other, and about 4 inches deep. Part of 

 this is shown in fig. 1 as K. The end came to within about 

 1J inches from the face of the grating G. 



Fig. 2 shows a section, which represents the construction 

 for about one-half of its length from the grating end, the 

 remainder being without the grooved slides (A). The tube 

 is shown in plan in fig. 3. 



The opening (B) was in that side of the tube nearest the 

 rail, and was necessary when the camera was close up to the 

 slit ; since the light from the slit would otherwise be cut off 

 from the grating by the angle of the tube coming between 

 them, as shown in fig. 3 in plan. 



An arrangement for wholly or partially closing this opening 

 at will was provided in the shape of a door sliding in the 

 grooves A, A. 



In order to close up the space below the tube K, inasmuch 



