October 30, 1891.] 



SCIENCE. 



243 



vice, if we could rely upon measurements of photographs of 

 the same stars taken at suitable intervals of lime. Professor 

 Pritchard, to whom is due the honor of having opened this 

 ' new path, aided by his assistants, has proved by elaborate 

 investigations that measures for parallax may be safely made 

 upon photographic plates, with, of course, the advantages of 

 leisure and repetition; and he has already by this method de- 

 termined the parallax for twenty-one stars with an accuracy 

 not inferior to that of values previously obtained by purely 

 astronomical methods. 



The remarkable successes of astronomical photography, 

 which depend upon the plate's power of accumulation of a 

 very feeble light acting continuously through an exposure of 

 several hours, are worthy to be regarded as a new revelation. 

 The first chapter opened when, in 1880, Dr. Henry Draper 

 obtained a picture of the nebula of Orion ; but a more impor- 

 tant advance was made in 1883, when Dr. Common, by his 

 photographs, brought to our knowledge details and exten- 

 sions of this nebula hitherto unknown. A further disclosure 

 took place in 1885, when the brothers Henry showed for the 

 first time in great detail the spiral nebulosity issuing from 

 the bright star Maia of the Pleiades, and, shortly afterwards, 

 nebulous streams about the other stars of this group In 

 1886, Mr, Roberts, by means of a photograph to which three 

 hours' exposure had been given, showed the whole back- 

 ground of this group to be nebulous. In the following year 

 Mr. Roberts more than doubled for us the great extension of 

 the nebular region which surrounds the trapezium in the 

 constellation of Orion. By his photographs of the great 

 nebula in Andromeda he has shown the true significance of 

 the dark canals which had been seen by the eye. They are 

 in reality spaces between successive rings of bright matter, 

 which appeared nearly straight owing to the inclination in 

 which they lie relatively to us. These bright rings surround 

 an undefined central luminous mass. I have already spoken 

 of this photograph. 



Some recent photographs by Mr. Russell show that the 

 great rift in the Milky Way in Argus, which to ths eye is 

 void of stars, is in reality uniformly covered with them. 

 Also, quite recently, Mr. George Hale has photographed the 

 prominences by means of a grating, making use of the lines 

 H and K. 



The heavens are richly but very irregularly inwrought with 

 stars, the brighter stars cluster into well-known groups upon 

 a background formed of an enlacement of streams and con- 

 voluted windings and intertwined spirals of fainter stars, 

 which becomes richer and more intricate in the irregularly 

 rifted zone of the Milky Way. 



We, who form part of the emblazonry, can only see the 

 -design distorted and confused; here crowded, there scattered, 

 at another place superposed. The groupings due to our posi- 

 tion are mixed up with those which are real. 



Can we suppose that each luminous point has no relation 

 to the others near it than the accidental neighborship of 

 grains of sand upon the shore, or of particles of the wind- 

 blown dust of the desert ? Surely every star, from Sirius 

 and Vega down to each grain of the light-dust of the Milky 

 Way, has its present place in the heavenly pattern from the 

 slow evolving of its past. We see a system of systems, for 

 the broad features of clusters and streams and spiral wind- 

 ings which mark the genei-al design are reproduced in every 

 part. The whole is in motion, each point shifting its posi- 

 tion by miles every second, though from the august magni- 

 tude of their distances from us and from each other, it is 

 ■only by the accumulated movements of years or of genera- 



tions th it some small changes of relative position reveal 

 themselves. 



The deciphering of this wonderfully intricate constitution 

 of the heavens will be undoubtedly one of the chief astro- 

 nomical works of the cominjr century. The primary task of 

 the sun's motion in space, together with the motions of the 

 brighter stars, has been already put well within our reach by 

 the spectroscopic method of the measurement of star motions 

 in the line of sight. 



From other directions information is accumulating: from 

 photographs of clusters and parts of the Milky Way, by 

 Roberts in this country, Barnard at the Lick Observatory, 

 and Russell at Sydney; from the counting of stars, and the 

 detection of their configurations, by Holden and by Back- 

 house; from the mapping of the Milky Way by eye, at Par- 

 sonstown ; from photographs of the spectra of stars, by Pick- 

 ering at Harvard and in Peru ; and from the exact portraiture 

 of the heavens in the great international star chart which 

 begins this year. 



I have but touched some only of the problems of the newer 

 side of astronomy. There are many others which would 

 claim our attention if time permitted: — the researches of the 

 Earl of Rosse on lunar radiation, and the work on the same 

 subject and on the sun by Langley : observations of lunar 

 heat with an instrument of his own invention by Mr. Boys; 

 and observations of the variation of the moon's heat with its 

 phase by Mr. Frank Very: the discovery of the ultra-violet 

 part of the hydrogen spectrum, not in the laboratory, but 

 from the stars: the confirmation of this spectrum by terres- 

 trial hydrogen in part by H. W. Vogel, and in its all but 

 complete form by Cornu, who found similar series in the 

 ultra-violet spectra of aluminum and thallium: the discovery 

 of a simple formula for the hydrogen series by Balmer: the 

 important question as to the numerical spectral relationship 

 of different substances, especially in connection with their 

 chemical properties; and the further question as to the origin 

 of the harmonic and other relations between the lines and the 

 groupings of lines of spectra (on these points contributions 

 during the past year have been made by Rudolf v. Kovesli- 

 gethy, Ames. Hartley, Deslandres, Rydberg, Griinwald, 

 Kayse and Runge, Johnstone Stoney, and others) : the re- 

 markable employment of interference phenomena by Pro- 

 fessor Michelson for the determination of the size, and distri- 

 bution of light within them, of the images of objects which 

 when viewed in a telescope subtend an angle less than that 

 subtended by the light-wave at a distance equal to the diam- 

 eter of the objective, — a method applicable not alone to 

 celestial objects, but also to spectral lines, and other ques- 

 tions of molecular physics. 



Along the older lines there has not been less activity; by 

 newer methods, by the aid of larger or more accurately con- 

 structed methods, by greater reflnempnt of analysis, knowl- 

 edge has been increased, especially in precision and minute 

 exactness. 



Astronomy, the oldest of the sciences, has more than re- 

 newed her youth. At no time in the past has she been so 

 bright with unbounded aspirations and hopes. Never were 

 her temples so numerous, nor the crowd of her votaries so 

 great. The British Astronomical Association formed within 

 the year numbers already about six hundred members. 

 Happy is the lot of those who are still on the eastern side of 

 life's meridian ! 



Already, alas! the original founders of the newer methods 

 are falling out, — Kirchhotf, Angstrom, D' Arrest. Secchi, 

 Draper, Becquerel, — but their places are more than filled: 



