After tea it was voted that the night was likely to be fine, so we all turned out. The night was uncertain: sometimes entirely clouded, sometimes partially, but objects were pretty well seen when the sky was clear: the latter part was much steadier. From the interruption by clouds, the slowness of finding with and managing a large instrument (especially as their finding apparatus is not perfectly arranged) and the desire of looking well at an object when we had got it, we did not look at many objects. The principal were, Saturn and the Annular Nebula of Lyra with the 3-feet; Saturn, a remarkable cluster of stars, and a remarkable planetary nebula, with the 6-feet. With the large telescope, the evidence of the quantity of light is prodigious. And the light of an object is seen in the field without any colour or any spreading of stray light: and it is easy to see that the vision with a reflecting telescope may be much more perfect than with a refractor. With these large apertures, the rings round the stars are insensible. The planetary nebula looked a mass of living and intensely brilliant light: this is an object which I do not suppose can be seen at all in our ordinary telescopes. The definition of the stars near the zenith is extremely good: with a high power (as 800) they are points or very nearly so—indeed I believe quite so—so that it is clear that the whole light from the great 6-feet mirror is collected into a space not bigger than the point of a needle. But in other positions of the telescope the definition is not good: and we must look to-day to see what is the cause of this fault. It is not a fault in the telescope, properly so-called, but it is either a tilt of the mirror, or an edge-pressure upon the mirror when the telescope points lower down which distorts its figure, or something of that kind. So I could not see Saturn at all well, for which I was sorry, as I could so well have compared his appearance with what I have seen before. I shall be very much pleased if we can make out what is the fault of adjustment, and so correct it as to get good images everywhere. It is evident that the figuring of the mirror, the polishing, and the general arrangement, are perfectly managed.
THE CASTLE, PARSONSTOWN, 1848, Aug. 30.
Yesterday we were employed entirely about the Great Telescope, beginning rather late. The principal objects had relation to the fault of definition when the telescope is pointed low (which I had remarked on the preceding night), and were, to make ourselves acquainted with the mechanism of the mirror's mounting generally, and to measure in various ways whether the mirror actually does shift its place when the telescope is set to different angles of elevation. For the latter we found that the mirror actually does tilt 1/4 of an inch when the tube points low. This of itself will not account for the fault but it indicates that the lower part is held fast in a way that may cause a strain which would produce the fault. These operations and reasonings took a good deal of time. Lord Rosse is disposed to make an alteration in the mounting for the purpose of correcting this possible strain.
THE CASTLE, PARSONSTOWN, 1848, Aug. 31.
The weather here is still vexatious: but not absolutely repulsive. Yesterday morning Lord Rosse arranged a new method of suspending the great mirror, so as to take its edgewise pressure in a manner that allowed the springy supports of its flat back to act. This employed his workmen all day, so that the proposed finish of polishing the new mirror could not go on. I took one Camera Lucida sketch of the instrument in the morning, dodging the heavy showers as well as I could; then, as the afternoon was extremely fine, I took another, with my head almost roasted by the sun. This last view is extremely pretty and characteristic, embracing parts of the mounting not shewn well in the others, and also shewing the Castle, the Observatory, and the 3-feet telescope. The night promised exceedingly well: but when we got actually to the telescope it began to cloud and at length became hopeless. However I saw that the fault which I had remarked on the two preceding nights was gone. There is now a slight exhibition of another fault to a much smaller extent. We shall probably be looking at the telescope to-day in reference to it.
THE CASTLE, PARSONSTOWN, 1848, Sept. 1.
Yesterday we made some alterations in the mounting of the great mirror. We found that sundry levers were loose which ought to be firm, and we conjectured with great probability the cause of this, for correction of which a change in other parts was necessary. The mirror was then found to preserve its position much more fixedly than before…. At night, upon trying the telescope, we found it very faulty for stars near the zenith, where it had been free from fault before. The screws which we had driven hard were then loosened, and immediately it was made very good. Then we tried with some lower objects, and it was good, almost equally good, there. For Saturn it was very greatly superior to what it had been before. Still it is not satisfactory to us, and at this time a strong chain is in preparation, to support the mirror edgeways instead of the posts that there were at first or the iron hoop which we had on it yesterday.
Nobody would have conceived that an edgewise gripe of such a mass of metal could derange its form in this way.
Last night was the finest night we have had as regards clouds, though perhaps not the best for definition of objects.
THE CASTLE, PARSONSTOWN,
1848, Sept. 2.