484 Proceedings of the Royal Irish Academy. 



XLIY. — On" Personal EreoIis in Asteoxomical Teansit Obsehya- 

 Tiois's. By JoHsr L.E. Dueyee, M.A., F.E.A.S., Astronomer at tlie Earl 

 of Eosse's Observatory. 



[Eead February 14tli, 1876.] 



The niimeroiis observations wHcb during tbis century have been 

 made at tbe astronomical observatories, have made astronomers discover 

 a cause of errors in tbe observations, -svbicb contributes to diminish the 

 accuracy which might otherwise be expected from our excellent in- 

 struments. It is the " personal error." And this error exists oftener 

 in transit observations than in any other, for which reason it becomes 

 of great importance in determinations of longitude, and in every com- 

 parison of the results of the determination of different observers as to 

 the meridian passage of a star. 



"While working at the Copenhagen Observatory, my attention was, 

 two years ago, turned to the study of this special subject, by the 

 prize question of the University, for the answer to which I received 

 the Gold Medal. When I, later, as astronomer at the Earl of Rosse's 

 Observatory, had been examining several catalogues of nebulae, and 

 even in these found the influence of the observers' individuality, I 

 was induced to extend my researches on the subject. Added to this, I 

 have been encouraged by several men of science, in whose opinion I 

 could not but place the highest confidence, who thought it would be 

 of some use to astronomers, if I published at once all the results of my 

 studies of the literature, and my examination of all observations, 

 which might contribute to the explanation of the phenomenon. In 

 the paper I have the honour to lay before the Eoyal Irish Academy, 

 •will be found many facts generally known, but I have thought it 

 advisable to treat the subject in its whole extension, in this way giv- 

 ing to astronomers a complete account of all the results which can be 

 derived at present, with respect to personal equations. 



Transits are now-a-days observed in two different ways, by the eye- 

 and-ear method, and by means of the chi'onograph, of which the former 

 method has been used since Bradley's time. Using this method, the 

 observer counts the seconds of the clock, aiad compares the distance of 

 the star from a vertical wire in the field of the telescope, at the last 

 second-beat before the transit over the wii'e, vrith its distance at the 

 first beat after the transit. In this way the observer judges what 

 fraction of a second has passed between the first second and the 

 transit.* In the chronographic method, the beats of the clock, by 

 means of an electric current, make marks on a strip of paper, which is 



* Sometimes, biit rarely, observers use anotber metbod, directly estimating tbe 

 interval bet'ween tbe second-beat and tbe transit (see, for instance, Briefwecbsel 

 zvriscben Gauss und Scbimiacber, i., p. 368). 



