iv Observations of Nebule and Clusters of Stars. 
defect that I could perceive. If I might venture a guess from recollection, I would say that about two 
out of three mirrors were good working mirrors, and that one out of three would bear any test that could 
be applied. By a good working mirror I mean one in which the instrumental defects fell very much 
below those arising from the state of the atmosphere on an average working night. 
The test usually applied was the performance of the mirror on a star of the 8th, 83, or 9th magnitude 
under a power of 750. Such stars are bright in the great telescope. They are usually seen as balls ot 
light, like small peas, violently boiling in consequence of atmospheric disturbance. If the night is good 
there will be moments now and then when the atmospheric disturbance will abruptly seem to cease for a 
fraction of a second, and the star is seen for an instant as the telescope really presents it. Itis by the 
appearance of such moments that the performance of the telescope must be judged. With the best ot 
your father’s mirrors that I saw, the appearance at such opportunities was like that of the light shining 
through a minute needle-hole in a card placed in front of a flame. I think any practical astronomer will 
agree with me in the opinion that mirrors of six feet in diameter that bore this test bordered very closely 
indeed on theoretical perfection. 
In order to test a mirror in the degree here described, it was necessary very carefully to adjust both 
the great and small mirrors so as to come accurately into their places when the telescope was raised to 
the altitude of the object on which it was to be tested. At other altitudes the defects of adjustment 
which arose from the yielding of the supports used so sensibly to impair the image as to defeat the object 
of fully testing the mirror, 
It is to be regretted now that we seldom looked at the class of objects that are known as test objects, 
since this would have facilitated a comparison between the best mirrors produced by Lord Rosse and 
other telescopes. But it could not on most occasions have been done without withdrawing from the 
current work of the Observatory valuable parts of the very best nights, whereas it was easy to get a 
suitable star at the altitude for which the adjustments had been made, and at some time of the night 
which would but little interfere with the regular work ; and the performance of the mirror on any such 
star was as complete an index of its quality as was required. There is, however, one test object, the 
companion of y Andromede, with the appearance of which in the telescope I am familiar. This star is 
conveniently situated near the zenith and upon a meridian not rich in nebulz. I could therefore look at 
it without interfering much with the Observatory work, and if I thought the mirror sufficiently good to 
be worth the additional test, [ used to make the necessary adjustments and scrutinize this object when- 
ever a good night came while it was within reach. The companion of y Andromedz is itself a very 
close double star, the interval being about half a second. The wsual appearance of the companion even 
with the best mirrors was a single bright mass of blue light, some seconds in diameter, and boiling 
violently. This appearance was due to atmospheric disturbance either inside the telescope tube, or 
somewhere within that great cylinder six feet in diameter and extending up through the atmosphere, 
which was filled by light from the star on its way to the observer’s eye. On the best nights on which I 
have seen this object, the patch of bright blue light was sufficiently small to be often oval, and on one 
night the state of the air was so good that it never enlarged so much as to appear round, but remained 
persistently oval. When this oval appearance presented itself either continuously or pretty often, the 
disturbance of the air would seem now and then suddenly to cease for perhaps half a second, and with 
Lord Rosse’s best mirrors the companion would then instantly become two very minute round specks of 
white light with an interval between, which, from recollection, I would estimate as equal to the diameter 
of either of them, or perhaps slightly less. They then looked like two very small needle-holes in a piece 
of card. This appearance would frequently recur, though it never lasted for more than a very short period 
on each occasion. It is, of course, the appearance which the instrument would have furnished uninter- 
ruptedly, if the state of the air had permitted. 
Believe me, &e., 
G. JOHNSTONE STONEY. 
The Earl of Rosse. 
