174 PHOTOGEAPHY IN ASTEOKOMICAL KESEAECH. 



and others, this property of the phite lias been used to record the 

 presence in the sky of vast regions of nebiHosity such as, we may 

 safely say, the eye would never have satisfactorily portrayed, not 

 altogether because of their faint ness (for in oue of his papers Pro- 

 fessor Barnard tells us that he was actually led to photograph such 

 a region because he had become vaguely conscious of it by eye obser- 

 vation), but because of their diffusion. It is noteworthy that these 

 beautiful photographs were taken with comparatively humble in- 

 struments, and we may be as yet only on the threshold of revelations 

 still to be made in this direction. 



Secondly, the photographic method represents a great advance in 

 facility of manipulation. A familiar example may be taken from 

 the domain of planetary discovery. In old time to recognize a new 

 object among numerous fixed stars it was necessary either labori- 

 ously ro map out the whole region, or to learn it by heart, so that it 

 was practically mapped in the brain. Now all this labor is avoided ; 

 two photographs of the same region, taken Avithout any strain on the 

 inemory or the measuring ability of the observer, can at a glance, by 

 a simple comparison, give the information that a strange object is 

 or is not present — information formerly obtained at so much cost. 

 Sometimes, indeed, the cost was so great that the information was 

 not obtained at all. For fifteen years Hencke searched without suc- 

 cess for a planet, and for nearly forty years after the discovery of 

 the first four small planets, in 1807, no further discoveries Avere 

 made, though hundreds were constantly crossing the sky, and a 

 dozen new planets are now found cA'ery year with little trouble. 



But though this instance of increase in facility is striking, it is far 

 from being the only one or even the most important. Wherever we 

 require a record of any kind, whether it be of the configuration of 

 stars, or of solar spots, or of the surface of the moon, or of a spec- 

 trum, the labor of obtaining it has been enormously reduced by the 

 photographic method. Think for a moment of Avhat this means in 

 the last instance only — think of the labor- involved in mapping one 

 single spectrum by eye observation; of the difliculty of settling by 

 such a method any doubtful question of the identity of certain lines 

 in the spectrum of a star. A few yc-ars ago Doctor JNIcClean an- 

 nounced that he had found oxygen in the star /? Crucis. Up to 

 that time this clement, so familiar to us on this earth, had appeared 

 to belong to us alone in the universe, for in no spectrum had its lines 

 been detected. The proof of its existence in fi Crucis depended on 

 the identity of a ninnbcr of lines in llie specli-um with some of those 

 of oxygen; and the meastu-es were sulliciently difficult on a photo- 

 graph, "so that for more than a year the scientific world refused to 

 pronounce a verdict. How long Avould the case have dragged on if 

 only visual measures had been possible? We may fairly doubt 



