350 SCIENTIFIC RECORD FOR 1885. 



" The individual values of Ja, computed by Dr. de Ball, differ quite 

 widely in some cases from the values obtained by Herr Nyr6n from the 

 same observations, and it seems impossible to discover the cause of 

 these differences. It is without doubt necessary to take into considera- 

 tion the influence which the motion of the solar system might exert upon 

 the aberration in the case of stars near the pole, as has been shown by 

 Dr. Seeliger." {Bull. Astron., July, 1885.) } 



The secular nutation of the earths axis. — " M. Folic, haviug deduced a 

 periodic formula for the secular variations in obliquity and in longitude, 

 applies the designation 'secular nutation of the earth's axis' to these 

 variations. Defining the normal equator as a plane, the inclination of 

 which to the ecliptic of a certain epoch is equal to the mean obliquity 

 of that epoch, and the intersection of which with tliis latter plane passes 

 at each instant through the mean equinox of that instant, he concludes 

 tliat, in virtue of the secular nutation of the earth's axis, the mean pole 

 describes around the normal pole, considered as fixed, an ellipse the 

 major axis of which, directed towards the pole of the fixed ecliptic (i e , 

 the mean ecliptic of the epoch), is sensible during several centuries. 

 The period of the secular nutation is about 30,000 years, differing little 

 from that of the precession on account of the slow motion of the node 

 of the ecliptic, which is only 8"'7 per annum. Assuming a uniform 

 value of 50" for the secular diminution of the obliquity, M. Folic com- 

 pares the results obtained from his formula with ancient observations 

 of the obliquity, and is thus led to announce that the empirical expres- 

 sion f,=0"-47G + 0"-000018^ for the annual diminution (where t is the 

 number of years from 1850) satisfies very closely the observations from 

 —250 to + 1487. This expression, however, gives a considerably greater 

 variation to the secular diminution of the obliquity than that wliich re- fl 

 suits trom Leverrier's researches." {Nature.) 



Precession tables. — In the Monthly Notices for December there will be 

 found tables by Mr. Rambaut for the reduction of Besscl's precessions 

 to those of Struve. 



Astronomical refraction. — M. Loewy has recently communicated to 

 the AcadSmie des Sciences an account of a method which he considers 

 to be especially adapted for determining the constant and other ele- 

 ments of refraction. The principle of the method is this : A glass 

 prism with silvered faces, forming a double mirror, is placed in front 

 of the object glass of an equatorial. By means of this arrangement 

 the images of two stars which are widely separated in the sky can be 

 brought close together in the focal plane of the telescope ; tlius one 

 star may be at the zenith and the other on the horizon, in which case 

 the measured distance between the images will be largely afiected by 

 refraction. After an interval of three or four hours, when the stars are 

 at an equal height above the horizon, and when, therefore, the effect of 



