Sept. 5, 1889] 



NATURE 



441 



TELESCOPES FOR STELLAR PHOTOGRAPHY} 

 I. 



I WILL ask you to remember that the subject of this 

 paper is not that of the proposed international pho- 

 tographic survey of the heavens itself, but of the instru- 

 ments which are to be used for that survey. No doubt a 

 communication on the survey itself, dealing with the 

 results aimed at, the conditions under which it is con- 

 sidered the best results may be arrived at, and the 

 general scheme under which it is proposed to measure, 

 define, and catalogue the position of the stars obtained, 

 would be more generally interesting than one on the 

 mere instrumental equipment ; but this part of the sub- 

 ject has already been amply and most efficiently dealt 

 with in lectures by Mr. Common and Dr. Gill at the Royal 

 Institution, while the subject of the instruments to be 

 used has only as yet been discussed in the more scientific 

 and technicaljournalsor proceedings of Societies ; besides 

 which, I may be pardoned for saying that I think when 

 actual work is commenced, the perfection of the instru- 

 mental equipment will be found to be a larger factor in 

 the attainment of success than has ever been the case in 

 any previous astronomical research. There is probably 

 nothing which surprises and excites the admiration of the 

 modern astronomer more than the work done in bygone 

 times by some of the older astronomers— work which was 

 the outcome of marvellous patience and ingenuity while 

 working with tools which would excite the pity and con- 

 tempt of the merest tyro in astronomy of the present day ; 

 but while I am by no means a sceptic as to the most im- 

 portant part of all telescopes being " the man at the small 

 end," I do believe that never before in any system of 

 astronomical observing has " the man at the small end " 

 been so completely dependent on the excellence of his 

 instrumental equipment, a disarrangement of any one 

 part of which would leave him utterly helpless. I trust, 

 therefore, you will bear with me while discussing and 

 describing a {t.\v of the more important mechanical 

 details of these instruments. 



You are aware, probably, that an International Con- 

 gress of Astronomers was held last year in Paris, and that 

 it was decided to start a number of Observatories, in 

 various parts of the world, each to take its share in pro- 

 ducing photographs of the heavens, to be afterwards used 

 in compiling a general chart, in which stars down to the 

 14th magnitude would be entered. 



Before we go further, it may be well to explain the 

 difference between this system of charting and the old 

 system, and what circumstances have led to this proposed 

 revolution in astronomical work. 



The system of mapping stars which has been used up 

 to the present time consists, as you are aware, in observing 

 all the stars seriatim, in a transit circle, or similar instru- 

 ment, and tabulating their declination {i.e. angular distance 

 north and south of the equator), or Polar distance, as 

 found by readings of the vertical centre, and their right 

 ascension {i:e. their distance measured on the equator 

 from an empirically fixed point in the heavens), as found 

 by the difference of time of the sidereal clock between 

 the passage of the star across the centre wire of the 

 telescope and that of the fixed point above referred to. 

 The essential point in the above system which I have to 

 direct your attention to is, that every single star has to be 

 examined by itself. 



The magnitude of the work of such a survey as the 

 Paris Congress has decided upon may be inferred from 

 the fact that there are probably some 20,000,000 stars to 

 be examined and catalogued. It is a good year's work 

 of a transit circle to tabulate 5000 stars ; supposing, 

 therefore, thirty or forty Obseivatories divide the work 

 between them, the survey would still occupy over 100 



' A Paper read by Sir Howard Grubb, F.R.S , befcre the Socieiy of Arts, 

 on April i8, 1888. 



years, and by that time the proper motion of the stars 

 would render a new survey necessary. 



Now, ever since photography has been practised, it has 

 been the dream of the astronomer to photograph the 

 heavens, and obtain, at one and the same time, the 

 positions not of one but of hundreds, or perhaps thousands, 

 of stars in each operation. But then, it may be asked, 

 why was not photography employed long since ? The 

 answer is, that until recently the amount of sensitiveness 

 obtained was not sufficient to allow of the fainter stars 

 impressing the plate within reasonable time, and con- 

 sequently it was found impossible to produce satisfactory 

 stellar photographs, except of the larger stars. 



Dr. Warren De la Rue was the first to point out the use 

 which might be made of photography for the purpose of 

 star-charting, and, as far back as i860 and 1 861, produced 

 photographs of star-clusters, &c. 



In 1864 or 1865, Rutherford, of New York, obtained 

 photographs of the larger stars, and while photographing 

 the moon with the great Melbourne telescope in 1867 I 

 took, for the purpose of adjustment, some photographs of 

 " Castor " ; but in an article which I wrote for the 

 "British Journal Photographic Almanack" in 1869, I 

 pointed out that for the development of celestial photo- 

 graphs we would have to look to the chemist and not to 

 the optician — in other words, that until we obtained more 

 suitable plates we could not expect much advance. This 

 has proved to be the fact, for with the advent of the 

 gelatine plates, and consequent increase of sensitiveness, 

 celestial photography received that impetus which has 

 eventuated in this proposition of an international photo- 

 graphic survey. I also pointed out in that same article 

 nineteen years ago that if by any possibility the exposures 

 could be reduced so far as to render the unsteadiness of 

 the image insensible — a rapidity which I said there was 

 no reason to suppose might not be obtained in the case of 

 the sun — we might expect great results, a prediction 

 which has since been verified by Prof Jansen's magnificent 

 pictures of the sun, with which you are all familiar. 



It is almost superfluous to remind you also of the mag- 

 nificent picture of the nebula in Orion by Mr. Common, an 

 example of celestial photography never yet surpassed. 



In 1882, Dr. Gill sent home to the Royal Astronomical 

 Society, and to the Paris Academy of Sciences, a photo- 

 graph of the great comet of that year, and called attention 

 to the large number of stars photographed on the same 

 plate. This photograph was obtained with an ordinary 

 photographic lens and camera attached to a clock-driven 

 equatorial. 



It was this, perhaps, that influenced the Paris Observa- 

 tory to construct the photographic telescope of about 13 

 inches aperture, 1 1 feet focal length, specially corrected 

 for the chemical rays, with which the splendid star charts 

 of the Messrs. Henry were obtained. Meanwhile, others 

 were not idle, and while Dr. Gill, through the munificence 

 of Mr. Nasmyth, obtained a 9-inch achromatic, which I 

 corrected of course for the chemical rays, Mr. Roberts, of 

 Liverpool, had a 20-inch reflecting telescope constructed 

 for the same purpose. 



It soon became evident that this new departure in the 

 system of star-charting was likely to be of very great 

 importance, and consequently an International Congress 

 of Astronomers assembled last year in Paris to discuss 

 the whole question. This Congress defined the size and 

 focus of the object glasses to be used, and laid down a 

 certain standard for the correction of the chromatic 

 aberration suitable to the nature of the work ; but left 

 almost all other points free for individual astronomers to 

 deal with as they thought best. In fact, the Congress 

 wisely defined only just such points as were necessary for 

 securing uniformity in the scale of the photographs, all of 

 which it is proposed shall be sent to some central bureau 

 to be examined, discussed, and made use of in compiling 

 the chart. 



