434 ASTROPHYSICS 



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 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 were made, 

 though hundreds were constantly crossing the sky, and 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, whether it be of the configuration 

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

 spectrum, 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 Dr. 

 McClean announced that he had found oxygen in the star /? Crucis. 

 Up to that time this element, so familiar to us on this earth, had ap- 

 peared to belong to us alone in the universe, for in no spectrum 

 had its lines been detected. The proof of its existence in /? Crucis 

 depended on the identity of a number of lines in the spectrum with 

 some of those of oxygen; and the measures were sufficiently difficult 

 on a photograph, so that for more than a year the scientific world re- 

 fused to pronounce a verdict. How long would the case have dragged 

 on if only visual measures had been possible? We may fairly doubt 

 whether a definite conclusion would ever have been reached at all. 

 By the sheer facility of the new method of work we have advanced 

 by leaps and bounds where we could only crawl before. 



Thirdly, there has been a great gain in accuracy from the intro- 

 duction of photography; and it is this quality which is above all 

 of value in the science of astronomy. 1 The wonderful exactness of 

 the photographic record may perhaps best be characterized by saying 

 that it has revealed the deficiencies of all our other astronomical 

 apparatus, - - object-glasses and prisms, clocks, even the observer 

 himself. 



It has almost been forgotten that in the early days the accuracy 



1 Two things may be measured on a photographic plate the position of an 

 object, or the density of the image; the former being an indication of its position 

 in the heavens, and the latter of its brightness. With the latter topic I do not 

 propose to deal, for the reason that it is in the hands of a much abler and more 

 experienced exponent; but the former alone will provide enough food for reflec- 

 tion. 



