Appendix. ill 
is afforded by (0.). Jf his visit occurred during the winter, and the wind was anything north of east, the 
hot air from the chimney [of the observatory] would certainly have interfered with vision. But be this 
as it may, my experience of many vears assures me, that this solitary instance must have been quite 
abnormal, and I think it reflects some discredit on our age that anyone should on such slight warrant 
have tried to blight the laurels of the dead. 
Believe me, &c., 
(Signed) T. R. ROBINSON. 
3, Palmerston Park, Dublin, 
April 2, 1878. 
My Dear Lorp, 
Before 1848, when your father entrusted me with the charge of his Observatory, he had acquired the 
power of sometimes giving to the mirrors of the great telescope, ¢.e., to mirrors of six feet in diameter, 
a figure which must, from their performance in the telescope, have approached the limit of theoretical per- 
fection. This great success, however, could only sometimes be attained, and he was occupied in frequently 
repeating the processes of regrinding and repolishing with a view to making the process more secure, and 
-also drawing up a body of rules which might be communicated to others, and which, if followed, would 
produce the desired result. 
To make it possible to carry on these experiments in the laboratory without putting a stop to the work 
of the Observatory, he had provided two mirrors for the great telescope, and two for the three-foot tele- 
“scope, so that one of each might be on the polishing machines in the laboratory, while the others were in 
use for observing. 
1t was very much easier to give a good figure to the three-foot mirrors, and Lord Rosse used to succeed 
in doing this almost every time he attempted it. It was also possible to test their performance in some 
degree without removing them from the laboratory. For the three-foot polishing machine was at the 
bottom of a tower with trap-doors in the roof and floors immediately over the mirror as it lay on the 
machine, so that the tower could be converted into a kind of telescope-tube, and the mirror tested on the 
spot with the help of diaphragms on a watch-dial raised above the tower upon a tall flagstaff.* If the day 
was cloudy, and if that time of the afternoon was chosen when the connexion currents were least, some 
information as to the performance of the mirror could be obtained in this way. Lord Rosse’s practice 
was to regrind, repolish, and in this way partially test, one of the three-foot mirrors, immediately before 
polishing a six-foot mirror. Although doing this occupied a few days, he believed that in the end it saved 
time, by preventing mistakes or oversights in the manipulation of the six-foot mirror. 
The testing of a six-foot mirror was a much more tedious operation. It could not be in any degree 
tested till it was carried over to the Observatory.t The mirror with the three tiers of levers on which it 
rested (and from which it could not be in the least disturbed), and with their carriage, weighed about 
seven tons. The whole had to be lifted from the polishing machine, and transferred to a large truck, on 
which it was slowly dragged by twenty-five or thirty men for a distance of about a quarter of a mile to 
the Observatory. Here the mirror that was in the telescope had to be taken out and the new one put in 
its place, the suspending band screwed up in the right degree to prevent inequality of pressure on the bed 
of levers, and the mirror adjusted. On account of the great weight all these operations had to be conducted 
with care to avoid shifting the mirror on its bed of levers. And after all this was done it was necessary 
to wait for a favourable night before the mirror could be tested. When tested, it sometimes turned out 
either that the figure given in polishing was not satisfactory, or that some pinch, which could not be got- 
ten rid of, had arisen in the subsequent manipulations. In other cases, however, the mirror bore the tests 
satisfactorily, and some mirrors bore the most severe tests that could be applied without betraying any 
* This method of testing is still employed. R. 
_t A mode of testing by tilting the mirror, when on the machine, towards an enameled watch face, fixed on a pole, was subsequently 
tried, but as carried out was not found sufficiently convenient. R. 
