IN THE MOUNTINGS OF THE TELESCOPES AT BIRR CASTLE. 
155 
The three-foot reflector. 
The three-foot reflector had up to the year 1874 been carried by an Altazimuth 
mounting constructed principally of wood, which, though possessing this advantage 
over that of the six-foot—that the instrument could be directed to any part of the 
visible heavens—was from its nature less convenient for general work, and did not pos¬ 
sess the steadiness of that of the six-foot, or of a mounting composed wholly of metal 
and masonry. The woodwork was in need of extensive repair, and it was decided to 
remount the instrument as an equatorial of such strength and solidity as to be as far as 
possible free from vibration even when exposed to a moderate force of breeze. 
The mounting now most in favour both for reflectors and refractors, which has been 
recently much improved in details by Mr. Grubb and others, and modified by the late 
Mr. Grubb in the case of the great Melbourne telescope by placing the upper bearing 
of the polar axis above instead of below its intersection with the declination axis, with 
the object of securing greater steadiness and ease of motion in so large a reflector, is 
that known as Fraunhofer’s. 
In it the tube of the telescope is situated at one extremity of the declination axis, 
and it is counterpoised round the polar axis by a weight at the other extremity of the 
declination axis. In following an object with a telescope mounted on this plan from 
near the eastern towards the western horizon it is, in general, necessary that the tube 
shall be “reversed” from one side to the other of the polar axis at or near to the 
meridian (usually the best position for observation). This is certainly a cause of 
considerable inconvenience in some classes of work. 
In the mounting about to be described “reversal” is also necessary, but only at the 
less advantageous positions for observation, the east and west points. The tube is 
balanced round the polar axis, and the only part which requires a counterpoise is the 
fork which carries the tube. 
The mounting adopted by Mr. Lassell for his two-foot and four-foot reflectors fur¬ 
nished the original idea, but with the kind assistance of Mr. Bindon B. Stoney, C.E., 
from whom many valuable suggestions were received, as also through the careful atten¬ 
tion given to the designing of the details of the various parts by Mr. W. G. Strype, C.E., 
who was the first to suggest the form of fork ultimately adopted, and superintended 
the execution of the whole, it is hoped that the mounting may be found well adapted 
for the work required.* 
In Mr. Lassell’s mounting the polar axis consists of a cone of boiler-plate, the 
apex of which is directed towards the south pole and rests in a step or bearing. The 
upper end is covered in with a plate, upon which are erected two standards which 
support the two extremities of the declination axis. The base of the cone turns upon 
antifriction rollers. 
* Mr. William Spence, of Cork Street, Dublin, was contractor for the work, masonry only excepted. 
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