450 



Prof. D. E. Hughes. 



[May 27, 



Bond in 1858 considered that these areas varied as the time of 

 exposure. Further investigation is required. 



III. In the last section the more important question is answered, 

 as to whether, on these modern photographic plates, where the times 

 of exposure are reckoned by hours and minutes rather than by 

 minutes or seconds, measures may be made of relative stellar 

 coordinates as exact and trustworthy as those derived from the best 

 astronomical instruments applied directly to the heavens. For the 

 purposes of this enquiry, resort is made to the same plates of the 

 Pleiades which furnished the results in Section I of this research. 

 The distances of twenty-five stars from Alcyone were measured with 

 the Oxford macro-micrometer: each measure on each of the four 

 plates was repeated the same number of times as Bessel measured the 

 same distances with his Konigsberg heliometer. The result of the 

 comparison is slightly in favour of the photographic measures ; the 

 average deviations of the repeated measures from the mean being in 

 the case of photographs 0*24" and with Bessel's heliometer measures 

 0-29." 



A remarkable circumstance fortunately occurred in the course of the 

 measures, in the case of one of the four plates ; unmistakable evidence 

 appeared that the photographic film had very slightly but mea- 

 surably shifted in the neighbourhood of three of the stars, but on no 

 other portions of that plate nor on any portion of any of the other 

 plates. This indicates the unadvisability of relying on any single 

 plate uncorroborated by others. 



The applicability therefore of this form of photography to the most 

 precise astronomical measurements seems to be established, even for 

 plates exposed for considerable times. I now propose to test the 

 method still further, by applying it to the determination of stellar 

 parallax. 



VIII. " Researches upon the Self-induction of an Electric 

 Current." By Professor D. E. Hughes, F.R.S. Received 

 May 24, 1886. 



Numerous researches have been made upon the self-induction of 

 coils of wire, and but few in relation to the influence exerted by the 

 nature and geometrical sectional form of the electrical conductor 

 when employed in straight wires as in those of a telegraph line with 

 the earth as a return, or those of a single wide loop where tho 

 distance of the return wire is sufficient to prevent any appreciable 

 effect from the mutual induction of separate portions of the wire upon 



