October 30, 1891.] 



SCIENCE. 



241 



nately towards and from our system, which would need for 

 its detection the fiducial positions of terrestrial lines com- 

 pared directly with them. 



For such observations the Potsdam spectrograph was well 

 adapted. Professor Vogel found that the bright star of Algol 

 did pulsate backwards and forwards in the visual direction in 

 a period corresponding to the known variation of its light. 

 The explanation which had been suggested for the star's 

 variability, that it was partially eclipsed at regular intervals 

 of 68.8 hours by a dark companion large enough to cut off 

 nearly five-sixths of its light, was therefore the true one. The 

 dark companion, no longer able to hide itself by its obscure- 

 cess, was brought out into the light of direct observation by 

 means of its gravitational effects. 



Seventeen hours before minimum, Algol is receding at the 

 rate of about 24^ miles a second, while seventeen hours after 

 minimum it is found to be approaching with the speed of 

 about 28|- miles. From these data, together with those of 

 the variation of its light, Vogel found, on the assumption 

 that both stars have the same density, that the companion, 

 nearly as large as the sun, but with about one-fourth his 

 mass, revolves with a velocity of about fifty-five miles a sec- 

 ond. The bright star, of about twice the size and mass, 

 moves about the common centre of gravity with the speed of 

 about twenty-six miles a second. The system of the two 

 stars, which are about three and a half millions of miles apart, 

 considered as a whole, is approaching us with a velocity of 

 2.4 miles a second. The great difference in luminosity of 

 the two stars, not less than fifty times, suggests rather that 

 they are in different stages of condensation, and dissimilar 

 in density. 



It is obvious that if the orbit of a star with an obscure 

 companion is inclined to the line of sight, the companion 

 will pass above or below the bright star, and produce no 

 variation of its light. Such systems may be numerous in 

 the heavens. In Vogel's photographs, Spica, which is not 

 variable, by a small shifting of its lines reveals a backward 

 and forward periodical pulsation due to orbital motion. As 

 the pair whirl round the common centre of gravity, the 

 bright star is sometimes advancing, at others receding. They 

 revolve in about four days, each star moving with a velocity 

 of about fifty-six miles a second in an orbit probably nearly 

 circular, and possess a combined mass of rather more than 

 two and a half times that of the sun. Taking the most proba- 

 ble value for the star's parallax, the greatest angular separa- 

 tion of the stars would be far too small to be detected with 

 the raost powerful telescopes. 



If in a close double star the fainter companion is of the 

 white-star type, while the bright star is solar in character, 

 the composite spectrum would be solar with the hydrogen 

 lines unusually strong. Such a spectrum would itself afford 

 some probability of a double origin, and suggest the existence 

 of a companion star. 



In the case of a true binary star the orbital motions of the 

 pair would reveal themselves in a small periodical swaying 

 of the hydrogen lines relatively to the solar ones. 



Professor Pickering considers that his photographs show 

 ten stars with composite spectra; of these, five are known to 

 be double The others are: r Persei, B, Aurigae, S Sagittarii, 

 31 Ceti, and /i Capricoi-ni. Perhaps ji Lyree should be added 

 to this list. 



In his recent classical work on the rotation of the sun, 

 Duner has not only determined the solar rotation for the 

 equator but for different parallels of latitude up to 75^. The 

 close accordance of his results shows that these observations 



are sufficiently accurate to be discussed with the variation of 

 the solar rotation for different latitudes which had been de- 

 termined by the older astronomical methods from the obser^ 

 vations of the solar spots. 



Though I have already spoken incidentally of the invalua- 

 ble aid which is furnished by photography in some of the 

 applications of the spectroscope to the heavenly bodies, the 

 new power which modern photography has put into the hands 

 of the astronomer is so great, and has led already, within 

 the last few years, to new acquisitions of knowledge of such 

 vast importance, that it is fitting that a few sentences should 

 be specially devoted to this subject. 



Photography is no new discovery, being about half a cen- 

 tury old: it may excite surprise, and indeed possibly suggest 

 some apathy on the part of astronomers, that though the 

 suggestion of the application of photography to the heavenly 

 bodies dates from the memorable occasion when, in 1839, 

 Arago, announcing to the Academic des Sciences the great 

 discovery of Niepce and Daguerre, spoke of the possibility 

 of taking pictures of the sun and moon by the new process, 

 yet that it is only within a few years that notable advances 

 in astronomical methods and discovery have been made by 

 its aid. 



The explanation is to be found in the comparative unsuita- 

 bility of the earlier photographic methods for use in the ob- 

 servatory. In justice to the earlier workers in astronomical 

 photography, among whom Bond, De la Rue, J. W. Draper, 

 Eutherfurd, Gould, hold a foremost place, it is needful to 

 state clearly that the recent great successes in astronomical 

 photography are not due to greater skill, nor, to any great 

 extent, to superior instruments, but to the very great advan- 

 tages which the modern gelatine dry plate possesses for use 

 in the observatory over the methods of Daguerre, and even 

 over the wet collodion film on glass, which, though a great 

 advance on the silver plate, went but a little way towards 

 putting into the hands of the astronomer a photographic sur- 

 face adapted fully to his wants. 



The modem siver-bromide gelatine plate, except for its 

 grained texture, meets the needs of the astronomer at all 

 points. It possesses extreme sensitiveness; it is always ready 

 for use; it can be placed in any position ; it can be exposed for 

 hours; lastly, it does not need immediate development, and 

 for this reason can be exposed again to the same object on 

 succeeding nights, so as to make up by several instalments, 

 as the weather may permit, the total time of exposure which 

 is deemed necessary. 



Without the assistance of photography, however greatly 

 the resources of genius might overcome the optical and me- 

 chanical difficulties of constructing large telescopes, the 

 astronomer would have to depend in the last resource upon 

 his eye. Now we cannot by the force of continued looking 

 bring into view an object too feebly luminous to be .seen at 

 the first and keenest moment of vision. But the feeble light 

 which falls upon the plate is not lost, but is taken in and 

 stored up continuously. Each hour the plate gathers up 

 3,600 times the light-energy which it received during the first 

 second. It is by this power of accumulation that the photo- 

 graphic plate may be said to increase, almost without limit, 

 though not in separating power, the optical means at the dis- 

 posal of the astronomer for the discovery of the observatioQ 

 of faint objects. 



.Two principal directions may be pointed out in which 

 photography is of great service to the astronomer. It ena- 

 bles him within the comparatively short time of a single 

 exposure to secure permanently with great cxacloess the 



