PHOTOGRAPHY IN ASTKONOMICAL RESEARCH. 177 



Although one or two ineritorious bogiiiiiings have been made, Avhich 

 have sufliced to show thai tliere are no insuperable difiiculties in the 

 way. up to the pi-esent moment no meridian instrument of repute 

 is in regular work using the phot()gra])hic method. And this fact 

 can not, after all, be completely ex})]ained by the reasons above men- 

 tioned. Opportunities for setting up costly new instruments do not 

 occur frequently in astronomy, but they do occur. In the last decade, 

 for instance, large transit circles have been set up both at (Ireenwich 

 and the Cajie of (iood Hope; but in neither instance has any attempt 

 been made to adopt the photographic metliod. The Washington 

 Observatory was reconstructed well within the period since the great 

 advantages of photograi^hy have been recognized, and yet not even 

 in the United States, the land of enter])rise, was a start then made in 

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

 day has probably been deferred by the stimulation of competing 

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

 first introduced into P^ngland the gas comi:)anies, 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 stai' ])ositions, 

 it would almost seem as if the devotees of the older visual methods 

 were compelled to improve their aj^paratus in order not to be left 

 wholly behind in the race. The registerin.g micrometer « was 



<' We have been aeeustoiued liitherto to iletennine the position of a star l).v 

 observing the instant wiien it crossed a ti.\ed winsluit it has long b(>en Icnown 

 that two different oliscrvcrs record s.\ sieiiiatically different instants — tliey Iiave 

 a personal e(iuation. Kei-ently we lia\e learned tiiat this iH>rsonal e(ination 

 varies with the brightness of the star observed, and with other circnnistances, 

 and to make the projter corrections for it has severely taxed our ingenuity and 

 involved much work. Before the invention of photography we might well berir 

 this with patience, since it seemed to be inevitable; but the i)hotographic plate 

 which is free from human errors, offers a wa.v of escape from all troubles, at 

 the expense, no doubt, of some little experimenting, but with every prosix'ct of 

 speedy success. Eye obsei'vation, which had i)orne this burden so long, nnist 

 get rid of it if it was to marcli alongside the untranmieled photograjjliic 

 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 wdth the star and registers its own movements. The register- 

 ing 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 e(|uation in this 

 human control. F>ut none manifests itself, proba!)ly for the reason that we no 

 longer liave two senses concerned, but only one. In recording tiie instant when 

 a star crosses a wire we employ either the eye and the ear. oi- the eye and the 

 sense of touch, and i)ersonal equation arises from the different coordination of 

 the two senses in different people. Hut in making the wire follow th(> star the 

 eye alone is concerned, and there is no longer any room for difference in " latent 

 period " or other coordination of two senses. 

 SM 1904 12 



