86 



KNOWLEDGE. 



[April 1, 1899. 



These examples show the advantages of trying new 

 forms of telescopes instead of duplicating those now exist- 

 ing. The Bruce telescope is well adapted to investigations 

 in which the focal length is small ; it will therefore be 

 interesting to try the effect of a great focal length. It is 

 proposed to build a telescope with an aperture of twelve to 

 fourteen inches, and a focal length of one hundred and 

 thirty-five or one hundred and sixty-two feet. This tele- 

 scope would probably be placed horizontally, and the star 

 reflected into it by means of a mirror, and the motion of the 

 Earth would be counteracted by moving the photographic 

 plate by clockwork. It would thus become a large hori- 

 zontal photo-heliograph. This method of mounting ,a 

 telescope for use on the stars was advocated by the 

 writer in 1881, and has been used here since then with 

 successive telescopes of two, four, and twelve inches aper- 

 ture. The instrument here proposed would be adapted to 

 investigations for which a great focal length would be 

 needed, as the latter would be more than a hundred times 

 the aperture. Several such investigations may be sug- 

 gested, any one of which, if successful, would amply justify 

 the construction of such an instrument. 



1. The Sun. The best instrument now in use for photo- 

 graphing the Sun — the horizontal photo-heliograph — is a 

 small instrument of this form. It is possible that, under 

 favourable atmospheric conditions, finer details on the Sun's 

 surface could be obtained with a large instrument than 

 have yet been photographed. It would be equally useful 

 in photographing the protuberances. Preparations must 

 soon be made for observing the solar ecHpse of May 28th, 

 1900. This instrument might be useful in photographing 

 the spectrum of the reversing lajer, and in showing the 

 details of the inner corona. 



2. The Moon. The images of the Moon obtained with 

 such a telescope would be more than a foot in diameter, 

 and even if printed without enlargement would probably 

 surpass the best photographs yet taken. The use of a 

 telescope of this form for photographing the Moon was 

 advocated by Prof. W. H. Pickering in 181)4 (" Harvard 

 Observ. Ann." XXXII., p. 110). It is possible that good 

 results could also be obtained with Jupiter, Saturn, and 

 perhaps Mars. 



3. Eros. This planet approaches the Earth so closely 

 that its parallax sometimes amounts to a minute of arc. 

 The next approach, in 1900, will be more favourable than 

 any other until 1927. Careful preparations should, there- 

 fore, be made for observing Eros when east and west of 

 the meridian, since the distance of the Sun can probably 

 be determined with more accuracy in this way than by any 

 method of observation yet attempted. As the distance of 

 the Sun is the unit to which all astronomical distances are 

 referred, the importance of its accurate determination can- 

 not be overstated. It is one of the great problems of 

 astronomy, which, though supposed in the eighteenth 

 century to have been solved, must probably be left to the 

 twentieth century for satisfactory solution. To determine 

 the parallax from the Transit of Venus in 1874, the prin- 

 cipal nations of the world sent expeditions to the most 

 remote regions. In all about eighty stations were occupied 

 at an expense of more than a million of dollars. 



4. The Fixed Stars. It is expected that the positions 

 of adjacent stars could be determined with this instrument 

 with an accuracy approaching that of the heliometer. ^f 

 so, it would have an important and permanent field of 

 work in charting the coarser clusters, the double stars, 

 and determining stellar parallax ; also in locating the 

 major planets, and the relative positions of the satellites 

 of .Jupiter and Saturn with an accuracy as yet un- 

 attained. 



The very moderate expenditure of five thousand dollars 

 to ten thousand dollars would permit this experiment to 

 be tried here, since we already have a portion of the 

 apparatus required. If successful, the name of the donor 

 would always be honourably associated with a new 

 departure iu one of the most important branches of 

 astronomy. 



February 11th, 1899. Edward C. Pickerino. 



WIDE ANGLE PHOTOGRAPHY IN ASTRONOMY. 



By E. Walter Maunder, f.r.a.s. 







NE of the first results of the great scheme of an 

 international photographic survey of the entire 

 sky was a rather curious one. There was a 

 sensible diminution in astronomical activity, at 

 any rate in England, apart from the more or less 

 routine work carried on by the great public observatories. 

 This would seem to have resulted from a vague, unspoken, 

 yet powerful impression that the great photographic 

 revolution which was in progress had rendered the older 

 methods of observation more or less obsolete, and in 

 particular had doomed the amateur astronomer to useless- 

 ness and extinction. 



In all probability, no one man ever formulated this im- 

 pression into a categorical statement, but for a time it 

 certainly had its effect. Nor was it without some plausi- 

 bility, for it was a new thing to see an astronomical 

 enterprise set on foot of so gigantic a nature that no single 

 observatory, however fully equipped and richly endowed, 

 could hope to deal with it. Nothing less than the associa- 

 tion of something like a score of the largest observatories 

 in the world could cope with it. 



The discouragement did not last long, and there were 

 some who never felt it. Dr. Isaac Roberts, for instance, 

 was very quick to realize that the international scheme 

 had by no means annexed the whole of the photographic 

 universe, and he promptly carved out a territory for 

 himself, which he has developed with a thoroughness and 

 a success which requires no setting forth to the readers of 

 Knowledge, who have had the privilege of studying so 

 many of his wonderful revelations of world systems in the 

 making. 



Here, then, are two great photographic surveys with 

 entirely different objects, carried out by very dissimilar 

 instruments, but both carried out so effectively as to render 

 any thought of competition quite out of the question, save 

 to extend Dr. Roberts' cluster and nebular studies to the 

 Southern Pole. 



Do these two great schemes cover the whole of possible 

 photographic research '? It might almost appear that with 

 some astronomers there had been a feeling that this 

 was indeed the case. If so, others have not been wanting 

 who have been most active in opening out new lines of 

 work. 



Amongst the pioneers of new photographic enterprise 

 two names are pre-eminent, those of Prof. Barnard and 

 Prof. Pickering. The impetus which the former has given 

 to the use of the portrait lens in astronomical photography 

 is well known, whilst the latter has been especially fertile 

 in devising new departments of work and new forms of 

 telescopes to carry them out. The present number of 

 Knowledge, for instance, has an article from him on the 

 use of photographic telescopes of extremely long focus. 

 But we would now specially refer to the recognition by 

 Prof. Pickering of the necessity, not only for one great 

 survey of the sky, carried out once, and once for all, but 



