712 Scientific Proceedings, Royal Dublin Society. 
Brashear, has a focal length of 21°5 feet; the ruled space 
is about 6 inches long, bearing 14,488 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. 
For various reasons we decided not to erect the spectrometer in 
a dark room, but determined rather to set it up ina room open to 
day- and sun-light, and to endeavour to devise the light-tight 
connexions between the working part of the instrument, which 
then become necessary for photographic, as well also as for eye, 
observations. 
A raised floor, 30 feet by 30 feet, and about 9 feet high, was 
built at one end of the Physical Laboratory of the University, the 
floor being supported on steel girders, the ends of which were built 
into two opposite side-walls of the laboratory, while the central 
portions were supported by steel columns resting on concrete 
foundations. 
The spectrometer was mounted on this raised floor in the fol- 
lowing manner :— 
Two red deal beams, each 12” by 3”, 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 carefully 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. 
