taken at the Lowell Observatory/ 489 



images on a plate of mine taken April 14 gave as a mean for 

 the ratio of the two : — 



polar diameter 1 



equatorial diameter 1*067 7' 



This makes the oblateness = = ■ g Q , 



and for the ellipticity = — ; — = titt,- 

 1 J b 14-8 



At the time of the photographs the phase amounted to 



9nn °^ ^ e ecma torial diameter. This was partially offset by 



the irradiation from the limb. Also the poles of the planet 

 are notably darker than the rest of the disk, causing a less 

 visible extension there, and this effect is increased by the 

 photographic plate. As the corrections thus work opposite 

 ways we may, to a sufficient degree of approximation, 

 consider them as neutralizing one another. And in this 

 connexion a caveat should be entered against measures 

 made with a colour screen by daylight, especially v\ hen the 

 planet shows a phase, unless these are subsequently corrected 

 for relative irradiation and loss by faintness on the ter- 

 minator. For in such cases measures may show less than 

 reality, thus erring in the opposite direction from what the 

 earlier measures, where irradiation was neglected, did. 



We will now compare this photographic value with what 

 micrometric and heliometer measures of the planet have 

 yielded. 



From the various determinations which have been made 

 of Jupiter's oblateness, we may perhaps select the following 

 as the best : — 



Date. Oblateness. 



J. D. Cassini 1691 1 : 15 Interesting as being 



the first ever made. 

 Bessel 1S33 1 : 15*73 Heliometer cor- 

 rected for phase. 



Secchi 1856 1 : 16-0(5 Micrometer. 



Kaiser 1S57 1 : 15-62 



Lohse 1873 1 : 15*80 



(Hough 1880 1 : 16*76) 



Barnard 1892-4 1 : 15*99 



AY. H. Pickering, Lowell 1894 1:16*11 

 Obs. 



(Lewis 1894-5 1 : 16*7) 



See 1900 1:15*53 



Mean of the last unbracketed seven. 1 : 15*83 



