42 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



The other nebula is of the gaseous order also, 

 as shown by Huggins. D' Arrest finds that its 

 light is changeable. He concludes by calling 

 into question the possibility of recognizing any 

 annual parallactic displacement of this nebula. 

 He points out that if we compare the present 

 place of the nebula in the heavens with that 

 indicated by Lalande, who very carefully ob- 

 served the nebula in 1790, we find that, unless 

 we adopt the improbable assumption that this 

 star is moving on a course parallel to the sun's, 

 and with equal velocity, we must infer that 

 the nebula is so far off that the sun's motion 

 lias not produced a perceptible displacement 

 of the nebula during the last eighty years. He 

 infers that the usual annual displacement can 

 scarcely be greater than 0.07, an arc which 

 no instrument yet constructed could indicate. 



Spectra of the Planets. After careful spec- 

 troscopic observations of the planets, Herr 

 Vogel announces results, briefly as follows : 

 Mercury shows the lines 0, D, E, F, and others ; 

 the red part of the spectrum being intense, 

 while the blue and violet are faint. Venus 

 exhibits an agreement of about 30 lines with 

 the solar spectrum, and is strong in the mag- 

 nesium lines, while the sodium lines are deli- 

 cate. Mars resembles the sun in 20 of the 

 principal lines, and Jupiter in 30. The spec- 

 trum of Uranus is characterized by absorption 

 bands, two of which coincide remarkably with 

 those produced by the atmosphere of the earth 

 as seen near the time of sunset. 



Observations in the Southern Heavens. Dr. 

 B. A. Gould, director of the observatory at 

 Cordova, furnishes occasional information of 

 his labors to the American Journal of Science. 

 In a letter dated September 4, 1872, he says 

 that, during the first fourteen months of his 

 sojourn at Cordova, he had completed the sur- 

 vey of the entire sky south of the 10th degree 

 of north declination, identified the stars from 

 their catalogues, and reduced their positions 

 to the mean equinox of 1872.0. Dr. Gould 

 finds that, on fine nights, stars not brighter 

 than 7.2 can be distinctly seen ; while a con- 

 siderable number, which have been seen and 

 recorded, are not above the 7.5 magnitude. 

 He has marked down some new variable stars, 

 one of which he especially mentions, since it 

 belongs to the northern hemisphere. It is the 

 star in Monoceros, No. 507 of the Hour VI. in 

 Bessel. Its apparent magnitude fluctuates be- 

 tween 6.2 and 7.3 in a period of about thirty- 

 one days. Dr. Gould hoped that the observa- 

 tory building would soon be completed, and 

 his researches in the clear and transparent 

 sky of Cordova enter upon an era of full ac- 

 tivity. 



Photographing Celestial OlJQcts. In his 

 opening address before Section A of the Brit- 

 ish Association, Mr. "Warren De La Rue gives 

 a review of late attempts to obtain lunar pho- 

 tographs for purposes of measurement. Pho- 

 tographs of the moon are free from distortion, 

 and therefore offer material of great value as 



the basis for a trustworthy map of the lunar 

 surface, and also for the solution of the prob- 

 lem of the moon's physical libration. Mr. Do 

 La Rue had seen an enlargement of a lunar 

 photograph taken with the great Melbourne 

 telescope, in which the primary image is 3^ 

 inches in diameter. He thinks that by such 

 means measurements on the surface of the 

 moon may be obtained more free from error 

 than by the use of the micrometer. He high- 

 ly commends the efforts of Prof. C. A. Young, 

 of Dartmouth College, to photograph the pro- 

 tuberances of the sun in ordinary daylight. 

 Prof. Young has obtained distinct reproduc- 

 tions of some of the double-headed promi- 

 nences ; and, although as pictures the impres- 

 sions may be of little value, yet there is every 

 reason to believe, now that the possibility of 

 the operation is known, that, with better and 

 more suitable apparatus, an exceedingly valu- 

 able and accurate record may be had. 



Prof. Young employed for the purpose a spectro- 

 scope ^containing seve'n prisms, fitted to a telescope 

 of 6i 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 photographic purposes, and only in the centre 

 yielded a true reproduction 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 applica- 

 tion of photography to astronomy ; but I must point 

 out also some cases where it has failed. Nebulae and 

 comets have not yet been brought within the grasp 

 of this art, although, perhaps, no branch of astrono- 

 my 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. Similar- 

 ly with reference to planets great difficulties still ex- 

 ist, 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 ultimately 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 Cran- 

 fprd of the occultation of Saturn by the moon some 

 time ago exhibits the ring of the planet in a manner 

 which holds out some promise for the future. 



In the reproduction of stars by photography, re- 

 cently undertaken by Mr. Rutherfurd, the objects to 

 be secured being so minute, special precautions were 

 found to be necessary in depicting them upon the 

 sensitive film, so that their impressions might be 

 distinguishable from accidental specks in the collo- 

 dion plate. To prevent any such chance of mistake, 

 Mr. Rutherfurd secures a double image of each lumi- 

 nary, the motion of the telescope being stopped for 

 a short time (half a minute) between a first and sec- 

 ond exposure of the plate ; so that each star is repre- 

 sented by two close specks, so to speak, upon the 

 negative, and is clearly to be distinguished by this 

 contrivance from any accidental speck in the film. 

 A map of the heavens is thus secured, very clear 

 though delicate in its nature, but yet one upon which 

 implicit reliance can be placed for the purposes of 



