472 



NA TURE 



[September 13, 190O 



photo-heliograph was installed at Kew. From 1858-72 a daily 

 record was maintained by the Kew photo-heliograph, when the 

 work was discontinued. Since 1873 the Kew series has been 

 continued at Greenwich, and is supplemented by pictures 

 from Dehra D(in in India and from Mauritius. The standard 

 size of the sun's disc on these photographs has now been for 

 many years 8 inches, though for some time a 12-inch series was 

 kept up. 



The first recorded endeavour to employ photography for 

 eclipse work dates back to 185 1, when Berowsky obtained a 

 daguerreotype of the solar prominences during the total eclipse. 

 From that date nearly every total eclipse of the sun has been 

 studied by the aid of photography. 



In i860 the first regularly planned attack on the problem by 

 means of photography was made, when De la Rue and Secchi 

 successfully photographed the prominences and traces of the 

 corona, but it was not until 1869 that Prof. Stephen Alexander 

 obtained the first good photograph of the corona, 



In recent years, from 1893 ""^^11 the total eclipse which 

 occurred last May, photography has been employed to secure 

 large-scale pictures of the corona. These were inaugurated in 

 1893 by Prof. Schaeberle, who secured a 4-inch picture of the 

 eclipsed sun in Chili : these have been exceeded by Prof. 

 Langley, who obtained a 15-inch picture of the corona in North 

 Carolina during the eclipse of May 1900. 



Photography also supplied the key to the question of the 

 prominences and corona being solar appendages, for pictures of 

 the eclipse sun taken in Spain in i860 terminated this dispute 

 with regard to the prominences, and finally to the corona in 

 1871. 



In 1875, in addition to photographing the corona, attempts 

 were made to photograph its spectrum, and at every eclipse 

 since then the sensitised plate has been used to record both the 

 spectrum of the chromosphere and the corona. The spectrum 

 of the lower layers of the chromosphere were first successfully 

 photographed during the total eclipse of 1896 in Nova Zembla 

 by Mr. Shackleton, though seen by Young as early as 1870, and 

 a new value was given to the wave-length of the coronal line 

 (wrongly mapped by Young in 1869) from photographs taken by 

 Mr. Fowler during the eclipse of 1898 (India). 



Lunar photography has occupied the attention of various 

 physicists from time to time, and when Daguerre's process was 

 first enunciated, Arago proposed that the lunar surface should 

 be studied by means of the photographically produced images. 

 In 1840 Dr. Draper succeeded in impressing a daguerreotype 

 plate with a lunar image by the aid of a 5-inch refractor. The 

 earliest lunar photographs, however, shown in England were 

 due to Prof Bond, of the United States. These he exhibited at 

 the Great Exhibition in 1851. Dancer, the optician, of Man- 

 chester, was, perhaps, the first Englishman who secured lunar 

 images, but they were of small size (Abney, " Photography"). 



Another skilful observer was Crookes, who obtained images of 

 2 inches diameter, with an 8-inch refractor of the Liverpool 

 Observatory. In 1852 De la Rue began experimenting in lunar 

 photography. He employed a reflector of some 10 feet focal 

 length and about 13 inches diameter. A very complete account 

 of his methods is given in a paper read before the British Asso- 

 ciation. Mr. Rutherfurd at a later date having tried an ii^-inch 

 refractor, and also a 13-inch reflector, finally constructed a 

 photographic refracting telescope, and produced some of the 

 finest pictures of the moon that were ever taken until recent 

 years. Also Henry Draper's picture of the moon taken 

 Sept. 3, 1863, remained unsurpassed for a quarter of a century. 



Admirable photographs of the lunar surface have been pub- 

 lished in recent years by the Lick Observatory and others. I 

 myself devoted considerable attention to this subject at one 

 time ; but only those surpassing anything before attempted have 

 been published in 1896-99 by MM. Loewy and Puiseux, taken 

 with the Equatorial Coude of the Paris Observatory, 



Star prints were first secured at Harvard College, under the 

 direction of W. C. Bond, in 1850 ; and his son, G. P. Bond, 

 made in 1857 a most promising start with double-star measure- 

 ments on sensitive plates, his subject being the well-known pair 

 in the tail of the Great Bear. The competence- of the photo- 

 graphic method to meet the stringent requirements of exact 

 astronomy was still more decisively shown in 1866 by Dr. 

 Gould's determination from his plates of nearly fifty stars in the 

 Pleiades. Their comparison with Bessel's places for the same 

 objects proved that the lapse of a score of years had made no 

 difference in the configuration of that immemorial cluster ; and 



NO. 16 II, VOL. 62] 



Prof. Jacoby's recent measures of Rutherfurd's photographs 

 taken in 1872 and 1874 enforce the same conclusion. 



The above facts are so forcible that no wonder that at the 

 Astrophotographic Congress held in Paris in 1887 it was 

 decided to make a photographic survey of the heavens, and now 

 eighteen photographic telescopes of 13 inches aperture are in 

 operation in various parts of the world, for the purpose of pre- 

 paring the international astrographic chart, and it was hoped 

 that the catalogue plates would be completed by 1900. 



Photography has been applied so assiduously to the discovery of 

 minor planets that something like 450 are now known, the most 

 noteworthy, perhaps, as regards utility being the discovery of 

 Eros (433) in 1898 by Herr Witt at the Observatory Urania, 

 near Berlin. 



With regard to the application of photography to recording 

 the form of various nebulae, it is interesting to quote a passage 

 from Dick's "Practical Astronomer," published in 1845, as 

 opposed to Herschel's opinion that the photography of a nebula 

 would never be possible. 



" It might, perhaps, be considered as beyond the bounds of 

 probability to expect that even the distant nebuloe might thus be 

 fixed, and a delineation of their objects produced, which shall 

 be capable of being magnified by microscopes. But we ought 

 to consider that the art is only in its infancy, and that plates of 

 a more delicate nature than those hitherto used may yet be pre- 

 pared, and that other properties of light may yet be discovered, 

 which shall facilitate such designs. For we ought now to set no 

 boundaries to the discoveries of science, and to the practical 

 applications of scientific discovery, which genius and art may 

 accomplish." 



It was not, however, until 1880 that Draper first photo- 

 graphed the Orion Nebula, and later by three years I succeeded 

 in doing the same thing with an exposure of only thirty-seven 

 minutes. In December 1885 the brothers Henry by the aid of 

 photography found that the Pleiades were involved in a nebula, 

 part of which, however, had been seen by myself {Monthly 

 Notices, vol. xl. p. 376) with my 3-foot reflector in February 

 1880, and later, February 1886 ; it was also partly discerned at 

 Pulkowa with the 30-inch refractor then newly erected. 



Still more nebulosity was shown by Dr. Roberts's photo- 

 graphs {ibid., vol. xlvii. p. 24), taken with his 20-inch reflector 

 in October and December 1886, when the whole western side 

 of the group was shown to be involved in a vast nebula, whilst 

 a later photograph taken by MM. Henry early in 1888 showed 

 that practically the whole of the group was a shoal of nebulous 

 matter. 



In 1 88 1 Draper and Janssen recorded the comet of that year 

 by photography. 



Huggins {Proc. Roy. Soc, vol. xxxii. No. 213) succeeded 

 in photographing a part of the spectrum of the same object 

 (Tebbutt's Comet 1 881, II.) on June 24, and the Fraunhofer 

 lines were amongst the photographic impressions, thus demon- 

 strating that at least a part of the continuous spectrum is due 

 to reflected sunlight. He also secured a similar result from 

 Comet Wells {Bnt. Assoc. Report, 1882, p. 442). 



I propose to consider the question of the telescope on the 

 following lines : (i) The refractor and reflector from their in- 

 ception to their present state. (2) The various modifications 

 and improvements that have been made in mounting these in- 

 struments, and (3) the instrument that has lately been introduced 

 by a combination of the two, refractor and reflector, a striking 

 example of which exists now at the Paris Exhibition. 



At a meeting of the British Association held nearly half a 

 century ago (1852) (Belfast) Sir David Brewster showed a plate 

 of rock crystal worked in the form of a lens which had been 

 recently found in Nineveh. Sir David Brewster asserted that 

 this lens had been destined for optical purposes, and that it never 

 was a dress ornament. 



That the ancients were acquainted with the powers of a mag- 

 nifying lens may be inferred from the delicacy and minuteness 

 of the incised work on their seals and intaglios, which could only 

 have been done by an eye aided by a lens of some sort. 



There is, how ever, no direct evidence that the ancients were 

 really acquainted with the refracting telescope, though Aristotle 

 speaks of the tubes through which the ancients observed distant 

 objects, and compares their effect to that of a well from the 

 bottom of which the stars may be seen in daylight (" De Gen. 

 Animalium," lib. v.) As an historical fact without any equivo 

 cations, however, there is no serious doubt that the telescope 

 was invented in Holland. 



