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_.