Dreyer — O/i Astronomical Transit Ohserrations. 



511 



that notHng "but his common personal error will influence his obser- 

 vations. 



After having considered the variation of the personal error during 

 ^shorter and longer intervals of time, and the influence of the eye-piece 

 on the estimation of a transit, we shall now turn our attention to two 

 circumstances which may have a similar influence, the magnifying 

 power and the apparent velocity of the motion of the star, varying 

 according to its polar distance. These two circumstances have often 

 been considered as perfectly identical. Bessel, for instance, tells us * 

 that he, acknowledging the importance of the question as to whether 

 the personal error varies according to the polar distance, had made 

 numerous experiments with several powers, whereby it was found to 

 be of no consequence whether he observed stars near the equator with 

 a power of 180, or with one of 66 ; and as stars with a N. P. D. not 

 smaller than 20°, seen with the former power, move as quickly or 

 more quickly, than equatorial stars seen with the latter, he concluded 

 that there was no fear of faults in his right ascensions arising from 

 the difference in polar distance. But this conclusion is not quite 

 certain, as C. Wolf has already remarked,! because the thickness of 

 the wire is increased by a higher power, whereby it may be difficult 

 to estimate its axis in like manner before and after the transit. It 

 seems, therefore, more correct to treat the two questions separately ; 

 moreover, neither have been examined sufficiently hitherto. With 

 respect to the magnifying power, the following tables show the few 

 results hitherto published : — 



I. Eye-and-Eak Method. 



Power. 



C. Wolf. 



34 

 43 



77 

 133 



+ 0^-163 

 + -151 

 + -111 

 + -104 



Power. 



F. Kaiser. 



P. J. Kaiser. 



Kam. 



van Hennekeler 



50 

 200 



- 0=^-094 



- -141 



+ 0<^-032 

 + -018 



+ 0^-188 

 + 0-160 



+ 0^-055 

 + 0-125 



* Konigs'berger Beobachtungen, -¥111., p. 8. 



t Annales de 1' Observatoire de Paris, Memoires, t. viii., p. 175. 



