174 PHOTOGRAPHY IN ASTRONOMICAL RESEARCH. 



and others, this property of the phite lias l)een used to record the 

 presence in the sky of A'list regions of neljulosity such as, we may 

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

 altogether because of their faintness {for in one of his papers Pro- 

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

 a region l>ecause he had become vaguely conscious of it by eye obser- 

 vation), but because .of their ditl'usiou. Il is noteworthy that these 

 beautiful photographs were taken with comparatively humble in- 

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

 still to be made in this directiou. 



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

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

 two photographs of the same region, taken without any strain on the 

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

 a pimple 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 tlie 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 were 

 made, though hundreds were constantly crossing the sky, ai;d a 

 dozen new planets are now found every 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, Avhether it be of the configuration of 

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

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

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

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

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

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

 in the spectrum of a star. A few years ago Doctor McClean an- 

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

 that time this element, so familiar to us on this eai-th, had appeared 

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

 bwn detected. 'Jlie i)roof of its existence in ft Crucis depended on 

 the identity of a number of lines in the spectrum with some of those 

 of oxygen; and the measures were sufficiently diffictdt on a })hoto- 

 gi-aph, so that for more than a year the scientific world refused to 

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

 only visual measures had been possible? We may fairly doubt 



