PHOTOGRAPHY IN ASTKONOMIt'AJ. KKSKARCH. 177 



Altlioiiali oiH' or two iiu ritoi'ious hcii-iniiino-s have been niadc, wliicli 

 liiuc siilliccd to shoAV that lIuMv ai'c no iiisiiix'rahlc dinicultics in the 

 way. n|) to the pri'siMit inomenl no mci-idian instrument of ivpnte 

 is in iH'iiulai" work usiuii' the photoiiraphic method. iVnd this fact 

 can not, after all, he comph'tely exj)hiined hy the reasons above inen- 

 (iont'd. ( )i)por(nnilies for setting' up costly new insti'unKMits do not 

 occiu- frecjuently in asti'onomy, but they do occur. In the last decade, 

 for instance, large transit ciirles have been set uj) both at (hvenwich 

 and the Cape of (jood lIo])e; but in neither instance has any attempt 

 b(>en made to adopt the photoi>"ra])hic method. The Washin<i;ton 

 ( )bservatory was reconstructed well within the period since the <ireat 

 advantages of photography have been recognized, and yet not even 

 in the United States, the land of enterprise, was a start then made in 

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

 day has probably been deferred by the stinndation of competing 

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

 iirst introdnced into P^ngland the gas companies, stimnlated 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 ncMv accuracy was attainable in measurement of star positions, 

 it woukl ahnost 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" was 



<■ We have been aceustonied hitherto to (leteriuine the position of a star by 

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

 that two different oi>servers record systoniatieally 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 nuicli work. Before the invention of photograr)hy we might well bear 

 lliis with patience, since it seemed to be inevitable; but the photograi)hic itlate 

 which is free from human errors, offers a wjiy of escape from all troubles, at 

 the expense, no doubt, of some little experimetiting. but with every prospect of 

 s]teedy success. Eye observation, which had borne this burden so long, must 

 got rid of it if it was t(» march alongside the untrauuiieled ])hotograpliic 

 method; and the suri)rising thing is that it has actually done so. The adojited 

 device is extremely simple: Hejdace the fi.xed wii-e wbidi tiie star crosses by a 

 wire which moves with the star and registers its own movements. The register- 

 ing is done automatically, lint the motion of the wire is controlled by tlie 

 observer, and then is still room for a new form of i)ersonal equation in this 

 human conti'ol. Kut none manifests itself, probably 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 tlie 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 aloue is concerned, and there is no longer any room for difference in " latent 

 period " or other coordination of two senses. 

 SM 190-1 12 



