Observations of Nebule and Clusters of Stars. 
APPENDIX. 
Since the foregoing paper was read, one or two friends, who in past years had had oppor- 
tunities of judging of the performance of the six-foot reflector, have expressed the opinion that 
the few remarks I had made relative to its defining power and to the criticisms of two or 
three astronomers thereon were insufficient, considering the importance of the subject. 
T have accordingly obtained from Dr. Romney Robinson permission to publish a letter he 
had written in 1871, in reply to my inquiries whether he had seen Mr. Proctor’s article in 
Frazer's Magazine, which he has kindly revised and made more concise. Mr. Johnstone Stoney 
also has been good enough to recount his experiences of former years with the six-foot in a 
letter which I subjoin. Dr. Robinson and Mr. Stoney had so many opportunities of witnessing 
the performances of the instrument during the period when the greatest attention was paid to 
the polishing process, that I do not think that we can have higher testimony on the subject. 
Atthe present moment when M. Tempel, being unable with his 11-inch object glass to see the 
details represented in our drawings of nebulze, announces his disbelief in the real existence of 
those details, ascribing them to the fancy of the observer, a few observations on the subject 
may possibly be a little more called for. 
1871, April, Armagh Observatory. 
My Dear Lorp Rosse, 
Not the least surprising part of Mr. Proctor’s strange attack on large reflectors, which appeared in 
Frazer's Magazine of Dec., 1869, is its depreciation of the six-foot, which goes so far as to assert 
that it is quite deficient as to definition, and only fit to condense light on the slit of a spectroscope. His 
only authority for such an assertion is that a foreign astronomer (not named) was shown Saturn in the 
six-foot, ‘and it might have been anything.” This might have happened, and yet not authorize Mr. P’s 
inference. A stream of heated air passing before the telescope, the agitation and hygrometric state of 
the atmosphere, and ‘any difference of temperature between the speculum and the air in the tube are all 
capable of injuring, or even destroying definition, though the speculum were absolutely perfect. 
The effect of these disturbances is in reflectors at least as the cube of their apertures ; and hence there 
are few hours in the year when the six-foot can display its full powers. 
My experience of it differs widely from that of the “foreign astronomer,” and you may think a few 
extracts from my old note-books worth preserving as a set-off against his. It is true that they contain 
few notices of ordinary test-objects ; for I seldom took stars except for level, or meridian, and mostly 
studied clusters andnebule. Many of these, especially the so-called planetary nebulz and the spiral ones, 
require very high defining power to bring out their details; they were wonderfully shown, but so few 
telescopes can show these at al/ that a reference to them would have no real significance except to a very 
few observers. Still there is enough to prove the excellence of the telescope. 
(a.) “First trial of it Feb. 11, 1845. Night unfavourable, but Sirius was very good with 400; 
yet on throwing it out of focus the ring of light was curdled, showing that the surface was not quite true, 
though the figure was good.” 
This your father had predicted. The speculum was, however, kept in the tube in hopes of a peep at 
Orion. It was repolished Mar. 3; but not very successfully, from some impurity in the ammonia used, 
which brought out a damask on its surface not easily got rid of. 
(0.) “Mar. 4. Speculum slightly dewed when first uncovered. When this passed off Regulus was 
small, neat, and round, with prodigious light ; y Leonis was very well defined ; with 360. After some hours 
work the thermometer sank to 23°, the stars flared and « Bootis was very indifferent.” 
TRANS. ROY. DUB. SOC., N.S. VOL. Il. 
