80 



KNOWLEDGE. 



[February 1, 1897. 



Eoberts about 1890, has added greatly to the powers of 

 nebular photography. By its means, Dr. Gill obtained, 

 more than a year ago, in twenty-four hours divided among 

 several nights, a grand picture of the " Keyhole " nebula 

 in Argo. Its extent and intensity are wholly due to the 

 patience of the sensitive plate, for the object is but feebly 

 actinic. The absence from it of a large, trident-shaped 

 structure, conspicuous in Sir John Herschel's drawing, 

 indicates a change which, although startling, cannot 

 readily be explained away. It would not, if real, be un- 

 exampled. Other nebula? have been found to vary in light, 

 notably Hind's in Taurus, which, after many vicissitudes, 

 was completely invisible with the Lick thirty-six-inch 

 refractor in 1895. 



The eye sees at once, or not at all ; the camera sees by 

 degrees : hence its effectiveness as an engine of discovery. 

 It brings into cognizance the 

 contents of depths of space 

 unsounded by the telescope, 

 as well as objects hidden by 

 their intrinsic faintness, or by 

 the quality of their radiations. 

 On November 16th, 1885, a 

 short, curved, nebulous train 

 attached to the star Maia, 

 unexpectedly emerged to 

 view in a photograph of the 

 Pleiades taken by the MM. 

 Henry ; and further investi- 

 gations by them and Dr. 

 Roberts showed the whole 

 cluster to be largely involved 

 in cosmic fog. Vast chaotic 

 masses of the same kind, 

 involving stars in the Milky 

 Way, have since declared 

 themselves on Barnard's and 

 Mas Wolf's negatives ; and 

 Prof. W. H. Pickering, using 

 aportrait-lens of insignificant 

 dimensions, secured in 1889, 

 from the summit of Wilson's 

 Peak in California, records 

 of a stupendous spiral, en- 

 veloping the major part of 

 Orion, and following a line 

 of incurvation, as if towards 

 a nucleus in the shining 

 nebula attached to the 

 Giant's Sword. 



The first photographic detection of a comet fell to the 

 share of Prof. Barnard, October 12th, 1892. Had it not 

 been for the intervention of the Willard lens, the object 

 mi^ht have altogether escaped notice. It proved to have a 

 period of six years, and seemed to have exhausted its vitality. 

 In asteroidal discovery by the same versatile method, Dr. 

 Max Wolf led the way. With the exceptions of the original 

 four— Certs, Juno, Pallas, and Vesta — all the minor planets 

 now known, to the number of nearly four hundred and 

 forty, have been introduced to our acqu^iintance during the 

 Queen's reign. They were captured telescopically by a 

 laborious compaiiscn of star-maps, until December 22nd, 

 1891, when No. 323 imprinted on a Heidelberg nega- 

 tive a trail, marking and measuring its motion. The 

 hint was at once taken, and the photographic registration 

 of these little bodies progresses apace. The supply of them 

 seems unlimited, and they are far more troublesome to keep 

 than to catch; so that the question arises whether they 

 are worth the cost of computation. But astronomers are 



Sir John IlEKSCnEi. 



reluctant to throw back into the sea of space any fish 

 that comes to their nets. 



The only photographic star-catalogue as yet existing is Dr. 

 Gill's " Cape Durchmusterung," the first volume of which, 

 containing one hundred and fifty-two thousand five hundred 

 entries, has just been issued from the press. In all, about 

 three hundred and fifty thousand stars, measured from the 

 Cape plates by Prof. Kapteyn, are enrolled in this compre- 

 hensive survey. Its construction, prompted by the comet 

 pictures of 1882, was designed as a preliminary to a still 

 greater work. On Dr. Gill's suggestion, an International 

 Congress met at Paris in April, 1887, and decreed the 

 construction of a photographic catalogue and chart which 

 should accurately represent the state of the whole heavens 

 at the close of the nineteenth century. This grandiose 

 idea bids fair to be adequately realized. At seventeen 



observatories scattered over 

 the globe, the necessary 

 materials are in rapid course 

 of accumulation. The cata- 

 logue will give the exact 

 places of perhaps a million 

 and a quarter stars. The 

 longer-exposed chart-plates 

 will record, by a current 

 estimate, some twenty mil- 

 lions. These will not, how- 

 ever, be boohed. They will 

 be inscribed in an atlas con- 

 sisting of copies on glass 

 of the original negatives. 

 Their identification at any 

 future time will in this way 

 be made possible. From 

 the study of the wonderful 

 mass of statistics thus pro- 

 vided, many curious disco- 

 veries may result ; knowledge 

 as to the laws of stellar 

 distribution can, at any 

 rate, hardly fail to be im- 

 proved. It is true that 

 difficulties in the way of 

 determining photographic 

 stellar magnitudes somewhat 

 impair this prospect. Visual 

 photometry has, by pro- 

 longed exertions — for which 

 Harvard College is especially 

 distinguished — been raised 

 to a satisfactory degree of precision ; but photographic 

 photometry is still in the empirical stage. The rank of 

 a star cannot yet be defined in terms of chemical action. 



Perhaps the most important to astronomy of the varied 

 faculties of the sensitive plate is its power of delineating, 

 in full detail and with exquisite delicacy, the spectra of the 

 heavenly bodies. It peiceives the ultra-violet rays to 

 which the eye is blind ; it almost ignores air tremors ; and 

 it affords lasting records in lieu of fugitive glimpses. 

 Dr. Huggins was a pioneer in the " spectrographic " art, 

 which led him, in 1879, to the memorable discovery of the 

 invisible section of the hydrogen-spectrum, as portrayed by 

 absorption in the analyzed light of Sirius, Vega, and some 

 other " white stars." The range of new lines thus unex- 

 pectedly revealed, form, with those previously known, a 

 harmonic series bound together by a numerical relation 

 known as " Balmer's Law"; and analogous series have 

 since been recognized in the spectra of various substances, 

 notably in that of helium. Their presence is of the utmost 



