Aug. 1st, 1887.] 



SCIENTIFIC NEAVS. 



141 



THE ROYAL INSTITUTION. 



The Applications of Photography in Astronomy. 



ON June 3rd, Mr. David Gill, LL.D., F.R.S., Her Majesty's 

 Astronomer at the Cape of Good Hope, delivered a lecture 

 on the above subject, and said : — 



On the 1 6th of April last there was held at Paris a congress, 

 attended by upwards of fifty astronomers and physicists, repre- 

 senting nearly every civilised nation in the world. It was con- 

 vened for the purpose of considering a scheme of international 

 co-operation in the work of charting the sky on a large scale. 

 Or, rather, its object was to obtain a series of pictures, which, 

 taken within a comparatively limited period of time, and with the 

 necessary precautions, would enable astronomers of tlie present 

 day to hand down to future generations a complete record of 

 the positions and magnitudes of all the stars in the heavens to a 

 given order of magnitude. The labours of that conference are 

 now concluded, certain important resolutions have been adopted, 

 and the way has been so far cleared for giving these resolutions 

 practical effect. 



The brothers Paul and Prosper Henry have been engaged 

 since 1871 in the construction of charts of the Ecliptic by the 

 older processes of observation, but when they reached that 

 portion of the heavens where the Milky Way crosses the 

 Ecliptic, the number of stars became so overwhelming that the 

 task of charting seemed almost too great for human patience and 

 skill. But, fortunately, the time had come when dry plate 

 photography could be called in to aid, and this aid was in the 

 hands of men singularly competent to develop such an oppor- 

 tunity to the fullest e.\tent. The brothers Henry had long 

 aspired to be not only distinguished practical astronomers, but, 

 following the traditions of Huyghens and the Herschels, they 

 desired also to be the artists of their own optical means. 

 Bound together by strong brotherly affection and common tastes, 

 gifted alike with practical talents of a high order, and with an 

 energy and determination of character that permit no obstacle to 

 success, these men thus happily united have devoted the spare 

 hours of their busy astronomical duties at the Paris Observatory, 

 first to the study of optics, and afterwards to the grinding and 

 polishing of lenses and specula, which have won for them a now 

 world-wide reputation as opticians of the highest rank. I had the 

 pleasure, a few weeks ago, of visiting the modest workshop 

 attached to their house at Montrougc, and I shall not soon forget 

 that visit, nor the many lessons, moral as well as practical, which 

 I learned. Every detail of their process of working has been 

 evolved by themselves ; they employ no assistant, and their 

 every appliance is simple and practical in a degree which I can 

 only compare with the simple and practical character of the men 

 who designed it. Such were the men above all others to develop 

 the application of photography to the charting of the heavens. 

 They had high appreciation of the value of the work which they 

 were about to undertake, they had the fullest knowledge of the 

 requirements of the case, and they had the practical skill which 

 enabled them to perfect the necessary apparatus. Their first 

 attempts were made with a telescope of six inches aperture (the 

 object-glass being specially ground for photographic work), and 

 the tube was temporarily adapted to an existing equatorial stand. 

 With an exposure of forty-five minutes, pictures of stars were 

 obtained to the 12th magnitude, in which the star discs were quite 

 round and sharply defined. Fully appreciating the beauty of 

 this result, and seeing its importance, Admiral Mouchez boldly 

 faced many administrative difficulties, and accepted without delay 

 the proposals of the brothers Henry to construct an object-glass 

 of thirteen inches aperture and about eleven feet focal length, as 

 well as the offer of M. Gautier to mount the same on a suitable 

 stand. The new instrument was mounted in May, 1885, and, 

 both from an optical as well as a mechanical point of view, it 

 was admirably adapted for its intended work, and the results 

 obtained by the brothers Henry, and rapidly published and cir- 

 culated by Admiral Mouchez, at once astonished and delighted 

 the astronomical world. 



Before the conference, a great many people — I will not say 

 astronomers — held that the chief object was to photograph as 

 many stars as possible, and simply preserve these plates or issue 

 photographic copies of them, so that astronomers of the future, 

 by merely comparing one of these originals or copies with a 

 similar photograph of the same part of the sky taken 50 or 100 

 years hence, would find out what stars had changed in position 

 or magnitude, or whether any new star had appeared. There is 



no doubt this was the view of the popular writers — it is very 

 easily understood, and it appeals very directly to the imagina- 

 tion. Such a project alone would no doubt have had great im- 

 portance, and would probably in the future have brought to light 

 a great many very interesting isolated facts. But for the broader 

 and more refined purposes of astronomy, for the discussion of 

 such great questions as the motion of the solar system in space, 

 the common movement of large groups of stars, the accurate de- 

 termination of precession, and the general refinement of astro- 

 nomy of precision, these mere pictures would have no value. It 

 was essential for these larger and more permanently important 

 ends that all data should be provided for the most refined deter- 

 mination of the absolute position of any star upon any plate. 

 This view was endorsed by the congress. The objects of the 

 survey of the heavens to be carried out were defined ultimately 

 thus ; — "To make a photographic chart of the sky for the pre- 

 sent epoch, and to obtain the data for determining the positions 

 and magnitudes of all the stars to the 14th magnitude," as that 

 magnitude is at present defined in France. 



It no doubt produces a strong effect on the imagination to be 

 told that astronomers are to be engaged on making charts of the 

 sky which will contain 60 or 100 millions of stars, or photo- 

 graphing stars on their plates which cannot be seen at all in the 

 most powerful telescopes. There is thus a strong temptation to 

 yield to this demand for sensation, to produce a few astonishing 

 plates with the loss of much precious time, and to sacrifice the 

 real progress of astronomy to the love of the marvellous. Be- 

 sides, what are you to do with pictures of 100 millions of stars 

 when you have got them ''. What would be the use of pictures 

 of all these stars, unless at some future time a sufficient number 

 of astronomers were to arise to compare similar photographs, 

 taken, say, one hundred years hence, with the photographs 

 taken in our day? I am happy to think that the number of 

 men who devote themselves to the pursuit of astronomy is on 

 the increase, but I have no desire that the number of men in 

 Great Britain who occupy themselves exclusively with astronomy 

 will ever correspond with that in the floating island of Laputa, 

 as described by Dean Swift, where all the men were exclusively 

 occupied with astronomy, and had to be flapped on the head 

 with little bladders containing parched peas to arouse them from 

 their abstract occupations. And yet, unless something of this 

 sort happens, I see no adequate prospect of the utilisation of 

 pictures of 100 millions of stars. 



The congress, therefore, very wisely limited there chart plates 

 to the 14th magnitude. But, as was well said by M. Bouquet 

 de la Grye, it was not necessary to summon fifty or sixty 

 astronomers to a congress to arrange for taking mere photographs 

 of stars — a number of photographers, provided with instruments 

 like the Henrys, could have done all that without a congress. It 

 was very strongly felt that the true niison d 'etre of the Conference 

 was to secure astronomical data, precise and exact as the 

 operations of astronomers should be. 



Accordingly they resolved that — 



" In addition to the duplicate series of plates giving all the 

 stars to the 14th magnitude, there should be a series of plates of 

 shorter exposure to insure a greater accuracy in the micrometric 

 measurement of the standard stars, and to render the construc- 

 tion of a catalogue possible. The plates intended for the forma- 

 tion of the catalogue shall contain all the stars to the nth 

 magnitude inclusive." That is to say, it was determined to 

 catalogue the absolute places of stars to the nth magnitude. 



But no photographic plate of itself gives us any information 

 about the absolute places of stars, though it gives the means to 

 determine the relative positions of the stars on the limited area 

 of each plate ; you must trust to the old-fashioned meridian 

 observations to determine the absolute places of the brighter 

 stars on each plate, and then measure the position of the fainter 

 stars relative to these standard stars. 



I need not enter into detail about the technical means which 

 are to be taken for eliminating the various sources of error, 

 such as contraction of the photographic film in course of de- 

 velopment, and so forth. The chart of stars to the 14th magni- 

 tude will be of importance for many purposes, such as the search 

 for minor planets, and the trans-Neptunian planet, for variable 

 stars, and for data as to the law of distribution of stars of the 

 higher order of magnitude. But I do not hesitate to say that 

 the work which astronomers of future generations will be most 

 grateful for, and which will most powerfully conduce to the 

 progress of astronomy, will not be the chart but the catalogue. 



