98 



KNOWLEDGE 



[Mat 1, 1894. 



mentioned, and which, though I first saw it with the large 

 eye-piece referred to, is well seen in the twelve-inch. The 

 southern sharply defined edge of the great bright area 

 in which is located the trapezium is distinctly bordered 

 by a dull red. There are other parts about it also that 

 show this colour. It is best seen in strong moonlight. 

 The three hundred and fifty eye-piece shows it also 

 distinctly. I am not prepared to say just what this 

 phenomenon is. I hardly think it is a telescopic effect, 

 because I have never seen it connected with any other of 

 the nebulffi. I first saw it some three or four years ago. 

 Mount Hamilton, California, 

 April 6th, 1894. 



Benun-ks by Mr. S. W. Burnham on the SMne subject. 



Eeferring to the note on the powers that can be usefully 

 used on large telescopes, which appears in the March 

 number of Knowledge, in which reference is made to 

 me and to my friend Mr. Barnard, I would say that the 

 highest power used by us on the thirty-six-inch refractor at 

 Mount Hamilton was two thousand six hundred. The focal 

 length of this eye-piece was given as one-fifth of an inch, 

 and its power was therefore originally called three thousand 

 three hundred, but a subsequent careful measurement 

 reduced this to two thousand six hundred. The other 

 powers in general use on the micrometer were one thousand 

 nine hundi-ed, one thousand five hundred, one thousand 

 seven hundred, five hundi-ed and twenty, and three 

 himdred and sixty. The lowest power was very rarely used 

 by me and, aside from the measurements of nebula, probably 

 not half a dozen times a year. The eye-piece next to that 

 was used in the measures of very wide couples, where a 

 large field was necessary, and occasionally for extremely 

 faint companions to bright stars, where the distance was 

 several seconds. Most of my measures were made with 

 powers of seven hundred, one thousand, and one thousand 

 five hundred. My practice has always been, in measur- 

 ing close pairs, to use the lowest power which will give 

 the separation necessary for good measures. The nights 

 are not frequent, even at Mount Hamilton, when the 

 highest power can be used to advantage ; but there are 

 many very close double stars which cannot be measured 

 with anything else, and one must wait for a favourable 

 opportunity for such work. I kept on my working list 

 most of the very close pairs known to be in rapid motion, 

 and those which for a long time had been regarded as 

 single. Where the distances were 0-15" or less, the 

 measures were made with the two highest powers, and the 

 closest of these were always measured with two thousand six 

 hundred. Occasionally I have examined a doubtful object, 

 under exceptionally favourable conditions, ^ath one lens of 

 the highest eye piece removed. This, of course, gave a higher 

 power — I do not know what it would be — but the use of a 

 single lens was only an experiment, as the micrometer 

 could not be used in that way. 



So far as my experience goes, these very high powers 

 are useless for anything but extremely close double stars. 

 I have looked at Saturn, Jupiter, and other planets many 

 times with high powers, and under no circumstances could 

 as much be seen in the way of surface details as with 

 moderate powers. For all ordinary purposes, I should sav 

 the range of useful eye-pieces would be within the limft 

 suggested— viz., about fifty to the inch of aperture. 



Of course, I always used the full aperture of the thirty-six- 

 inch refractor in all my double star work. After using a large 

 instrument, one does not voluntarily take a smaller one, 

 nor reduce the aperture for any visual purpose. Nothing 

 is gained with any condition of the air by cutting down 

 the objective ; but there are some nights so poor— and I 



have never seen worse ones than we had at Mount Hamilton 

 at times — when a small instrument is as good as a large 

 one, but these are occasions when no one would think of 

 attempting to make any measures. 

 Chicago. 



ON THE MOUNTING OF LARGE REFLECTING 

 TELESCOPES. 



By Sir Howaed Grubb, F.R.S., &c., &c. 



WHATEVER may be said as to the relative merits 

 and demerits of reflectors and refractors, 

 there can be very little doubt that, for the 

 present, we must look to reflectors as offering 

 the most likely form by which to increase, to 

 any very large extent, the light-grasping power of our 

 telescopes. The difficulties in obtaining optical glass have 

 of late years been considerably reduced, and at the present 

 time discs of thirty inches diameter are not much more 

 diflicult to obtain than discs of half that diameter were 

 twenty years ago. There does not, however (in spite of 

 recent advances), seem to be much chance of obtaining 

 discs of optical glass which will enable us to construct an 

 objective which would equal in light-grasping power such 

 an instrument as Lord Eosse's six-foot reflector, which 

 has been in existence for half a century. 



It seems strange that fifty years have been allowed to 

 elapse without any attempt being made to develop the 

 power of the reflector. This is not due to any optical 

 imperfections, or any great difficulty in making the optical 

 parts perfect, as is clearly proved by the splendid work 

 accomplished with reflectors by Draper, De la Rue, Common, 

 Roberts and others. Modern astronomical research, 

 however, requires that all instruments should be mounted 

 with an accuracy and delicacy which it was scarcely 

 possible to obtain, even if it had appeared necessary, at 

 the time when Lord Rosse's six-foot mirror was mounted. 

 Whenever the reflecting telescope has been mounted with 

 the rigidity and stability required for stellar photographic 

 work, when the clock-driving has been sufficiently accurate, 

 and when the correcting slow motions have been sufficiently 

 delicate, the mirror has held its own against the object- 

 glass ; and, at the present day, if the six-foot mirrors of 

 the late Lord Rosse were properly mounted on equatorials 

 which would carry them with the accuracy required for 

 modern work, it cannot be doubted that they would do 

 magnificent work in the hands of present-day astronomers. 

 There appears to be some probability that an attempt 

 will be made by our neighbours on the other side of the 

 English Channel to make a distinct advance in this 

 direction. In view of the rumours of a ten-foot reflector to 

 be constructed and mounted for the great exhibition in 

 Paris in 1900, it may be interesting to consider some of 

 the difficulties that will probably arise in the construction 

 of such an instrument, and how such difficulties might 

 possibly be overcome. 



I do not propose to deal here with the optical difficulties, 

 feeling quite satisfied that they can be successfully dealt 

 with,if a disc of glass of sufficient size can be obtained; and 

 if not, then by the use of speculum metal. Dr. Common, 

 who has a most intimate Icnowledge of the difficulties to be 

 overcome in making such large instruments, is of opinion 

 that a silver-on-glass mirror of eight or nine feet aperture 

 is now possible. 



It is, however, to the mechanical difficulties of con- 

 struction that I wish to address myself, as I believe, up to 

 this time, this matter has not been very seriously treated. 

 The problem is indeed one of enormous difficulty, and the 

 working out of the details wiU be a serious matter. 



