Au^. 15, 1872J 



NATURE 



315 



probably change their appearance every moment, even at the 

 same station. If they are cislunar, the same appearances could 

 not be recorded at distant stations. It is universally admitted 

 that proof of the invariability of these markings, and especially 

 of their identity as seen at widely separated stations, would 

 amount to a demonstration of their extra-terrestrial origiru Eye- 

 sketches cannot be depended on ; the drawings made by persons 

 standing side by side differ often to an extent that is most per- 

 plexing. Now photographs have, undouljtedty, as yet failed to 

 catch many of the faint markings and delicate details ; but their 

 testimony, as far as it goes, is unimpeachable. In 1870, Lord 

 Lindsay at Santa Maria, Prof. Winlock at Jerez, Mr. Brothers 

 at Syracuse, obtained pictures some of which, on account partly 

 of the unsatisfactory state of the weather, could not compare with 

 Mr. Brothers' picture obtained with an instrument of special 

 construction ; * but all show one deep rift especially, which 

 seemed to cut down through both the outer and inner corona 

 clear to the limb of the moon. Even to the naked eye it was one 

 of the most conspicuous features of the eclipse. Many other 

 points of detail also come out identical in the Spanish .and Sicilian 

 pictures ; but whatever doubts may have still existed in regard of 

 the inner corona were finally dispelled by the pictures taken in 

 India, in 1871, by Col. Tennant and Lord Lindsay's photogra- 

 phic assistant, Mr. Davis. 



None of the photographs of 1S71 show so great an extension 

 of the corona as is seen in Mr. Brothers' photograph, taken at 

 Syracuse in 1S70 ; but, on the other hand, the coronal features 

 are perfectly defined on the several pictures, and the number of 

 the photographs renders the value of tlie series singularly great. 

 The agreement between the views, as well those taken at different 

 times during totality as those taken at different stations, fully 

 proves the solar theory of the inner corona. We have in all the 

 views the same extensive corona, with persistent rifts similarly 

 situated. Moreover there is additional evidence indicated by the 

 motion of the moon across the solar atmospheric appendages, 

 proving in a similar manner as in 1S60, in reference to the pro- 

 tuberances, the solar origin of that part of the corona. 



It will be well here to mentioa a difficulty which occurs in 

 recording the fainter solar appendages, namely the encroachment 

 of the prominences and the corona on the lunar disc when the 

 plates have to be over-exposed in order to bring out the faint 

 details of the corona. It is satisfactory to find that whenever a 

 diflSculty arises it can be mastered by proper attention. Lord 

 Lindsay and Mr. Ranyard have successfully devoted themselves 

 to experiments on the subject. They tested whether reflections 

 from the back surface of the plate played any part in the produc- 

 tion of the fringes : for this purpose plates of ebonite and the 

 so-called nonactinic yellow glass were prepared ; and it was 

 immediately found that the outer haze hail completely disappeared 

 in the photographs taken on ebonite, while on the yellow glass 

 plates it is much fainter than on ordinary white glass plates. By 

 placing a piece of wetted black paper at the back of an unground 

 plate, the outer haze was greatly reduced ; but by grinding both 

 the back and the front surfaces of a yellow glass plate, and 

 covering the back with a coating of black varnish, it was rendered 

 quite imperceptible, thus showing the greatest part of the so- 

 called photographic irradiation to be due to reflection from the 

 second surface. 



In connection with the solution of the most prominent ques- 

 tions connected with the solar envelopes, it may not be without 

 great interest to allude to another point conclusively decided 

 during the last annular eclipse of the sun, observed by Mr. Pog- 

 son on the 6th of June of this year, as described by him in a letter 

 to Sir G. B. Airy. In 1870 Prof. Young was the first to ob- 

 serve the reversal of the Frauenhofer lines in the stratum closest 

 to the sun. Now, in 1871 doubts were thrown upon the subject. 

 It appears that the reversed lines seem to have been satisfactorily 

 observed by Captain Maclear at Bekul, Colonel Tennant at 

 Dodabetta, and Captain Fyers at Jaffna. The observations of 

 Pringle at Bekul, Respighi at Poodocottah, and Pogson at 

 Avenashi were doubtful, while Mosely at Trincomalee saw 

 nothing of this reversal, which is, according to all accounts, 

 a most striking phenomenon, although of very short duration. 

 Mr. Lockyer missed it by an accidental derangement of the tele- 



* Mr. Erothers had, in 1S70, the happy idea to employ a so-called rapid 

 rectilinear photographic lens, made by Dallmeyer, of 4 inches aperture and 

 30 inches focal length, mounted equatorially, and driven by clockwork ; and 

 he was followed in this matter by both Col. Tennant and; Lord I-indsay in 

 1871. The focal image produced^ however, is far too small d^^ of an iuLh, 

 about) ; therefore it will be desirable in future to prepare lenses of similar 

 coastructlon, but of longer focal length and corresponding aperture. 



scope. The reversal and the physical deductions from it are 

 lilaced beyond doubt by Mr. Pogson's observation of tlie annular 

 eclipse oil June 6. At the first internal contact, just after a peep 

 in the finder had shown the moon's limb lighted up by the 

 corona, he saw all the dark lines reversed and bright, bat for less 

 than two seconds. The sight of beauty above all was, however, 

 the reversion of the lines at the breaking-up of the limb. The 

 duration was astonishing— five to seven seconds ; and the fading- 

 out was gradual, not momentary. This does not accord with 

 Captain Maclear's observations in 1870, who reports the disap- 

 pearance of the bright spectrum as " not instantly, but so 

 rapidly that I could not make out the order of their going." 

 Prof. Young again says that "they flashed out like the stars 

 from a rocket-head." But discrepancies in this minor point may 

 be accounted for by supposing dilTerences in quietude of that por- 

 tion of the sun's limb last covered by the moon. 



The mention of the solar appendages recalls to mind another 

 instance in which photography has befriended the scientific in- 

 vestigator. I allude to the promising attempt which has been 

 made by Prof. Young to photograph the protuberances of the 

 sun in ordinary daylight. A distinct reproduction of some of the 

 double-headed prominences on the sun's limb was obtained ; and 

 although as a picture the impression may be of little value, still 

 there is every reason to believe, now that the possibility of the 

 operation is knowm, that with better and more suitable apparatus 

 an exceedingly valuable and reliable record may be secured. 

 Prof. Young employed for the purpose a spectroscope containing 

 seven prisms, fitted to a telescope of 61-inch aperture, after the 

 eye-piece of the same had been removed. A camera, with the 

 sensitive plate, was attached to the end of the spectroscope, the 

 eye-piece of which acted in the capacity of a photographic lens, 

 and projected the image on the collodion film. The exposure 

 was necessarily a long one, amounting to three minutes and a 

 half. The eye-piece of the spectroscope was unsuitable for pho- 

 tographic purposes, and only in the centre yielded a true repro- 

 duction of the lines free from any distortion. A larger telescope, 

 with a suitable secondary magnifier, will be required, in order to 

 secure a more defined image. 



I have hitherto spoken of the successful applications of photo- 

 graphy to astronomy ; but I must point out also some cases 

 where it has failed. Nebulce and comets have not yet been 

 l>rought within the grasp of this art, although, perhaps, no 

 branch of astronomy would gain more if we should hereafter 

 succeed in extending to these bodies that mode of observing 

 them. There is theoretically, and even practically, no real limit 

 to the sensitiveness of a plate. Similarly with reference to planets 

 great difficulties still exist, which must be overcome before their 

 phases and physical features can be recorded to some purpose by 

 photography ; yet there is great hope that the difficulties may be 

 u'timately surmounted. The main obstacle to success arises 

 from atmospheric currents, which are continually altering the 

 position of the image on the sensitive plate ; the structure of the 

 sensitive film is also an interfering cause for such small objects. 

 A photograph taken at Cranford of the occultation of .Saturn by 

 the moon some time ago exhibits the ring of the planet in a 

 manner wliich liolds out some promise for the future. 



The niijon, on the other hand, has been for some time past 

 very successfully photographed ; but no use has hitherto been 

 made of lunar photographs for the purposes of measurement- 



The photographs of the moon are free from distortion, and 

 offer therefore material of incalculable value as the basis of a 

 selenograpliic map of absolute trustworthiness, and also for the 

 solution of the great problem of the moon's physical libration. 

 This question can be solved with certainty by a series of syste- 

 matic measurements of the distance of definite lunar points from 

 the limb. Mr. Ellery, Director of the Observatory of Melbourne, 

 has sent over an enlargement of a lunar photograph taken with 

 the Great Melbourne Telescope, in which the primary image is 

 3 i\ inches in diameter. Such lunar negatives would be admi- 

 rably adapted for working out the problem of ,the physical 

 libration, and also for fundamental measurements for a seleno- 

 graphic map ; the more minute details, however, would have to 

 be supplied l>y eye-observations, as the best photograph fails to 

 depict aU that the eye sees with the help of optical appliances. 

 On the other h.and, selenographic positions would be afforded 

 more free from error than those to be obtained by direct micro • 

 metrical measurements. 



Although, as I have stated, I do not contemplate passing in 

 review recent discoveries in astronomy, I must not omit to call 

 your attention to some few subjects of engrossing interest. First, 



