437 



in a direction in which it is certain that we must some day travel. 

 That day has probably been deferred by the stimulation of com- 

 peting methods which a new one brings with it. When electric light 

 was first introduced into England, the gas companies, stimulated 

 by the stress of competition, adopted a new and improved form of 

 light (the incandescent gas) which put them at a much less serious 

 disadvantage compared with their new rival. So when photography 

 began to show what new accuracy was attainable in measurement 

 of star-positions, it would almost seem as if the devotees of the older 

 visual methods were compelled to improve their apparatus in order 

 not to be left wholly behind in the race. The registering micrometer 1 

 was produced by Messrs. Repsold, with the astonishing result that 

 the troubles from personal equation, which have so long been a diffi- 

 culty in all fundamental work, have practically disappeared. 



This beautiful invention has placed the eye once more in a posi- 

 tion actually superior to the photographic plate; for with the eye we 

 can observe stars in daylight, and so secure information of great 

 importance, whereas no photographic method of doing this has, as 

 yet, been devised. And there is also the fact that for faint stars a 

 long exposure would be required for what the eye can accomplish 

 in a few seconds. 



Thus in one or two astronomical channels the effects of the rising 

 tide of photography have scarcely yet been felt; but into all the 

 others it has swept with ever-growing force. Looking back over the 

 thirty years of advance, we may be well satisfied. With more funds, 

 and especially with more men, no doubt more could have been done: 

 let us even admit that we might have done better with the same 

 funds and the same limited staff. But on the whole we have been 

 fortunate. At a critical time, when we might have felt the want of 



1 We have been accustomed hitherto to determine the position of a star by 

 observing the instant when it crossed a fixed wire; but it has long been known 

 that two different observers record systematically different instants they have 

 a personal equation. Recently we have learned that this personal equation varies 

 with the brightness of the star observed, and with other circumstances, and to 

 make the proper corrections for it has severely taxed our ingenuity and involved 

 much work. Before the invention of photography, we might well bear this with 

 patience, since it seemed to be inevitable; but the photographic plate, which is 

 free from human errors, offers a way of escape from all troubles at the expense, 

 no doubt, of some little experimenting, but with every prospect of speedy success. 

 Eye-observation, which had borne this burden so long, must get rid of it if it was 

 to march alongside the untrammeled photographic method; and the surprising 

 thing is that it has actually done so. The adopted device is extremely simple: 

 replace the fixed wire which the star crosses by a wire which moves with the star 

 and registers its own movements. The registering is done automatically; but the 

 motion of the wire is controlled by the observer, and there is still room for a new 

 form of personal equation in this human control. But none manifests itself, prob- 

 ably for the reason that we no longer have two senses concerned, but only one. 

 In recording the instant when a star crosses a wire we employ either the eye and 

 the ear, or the eye and the sense of touch; and personal equation arises from the 

 different coordination of the two senses in different people. But in making the 

 wire follow the star, the eye alone is concerned, and there is no longer any room 

 for difference in " latent period" or other coordination of two senses. 



