136 Corrections in Right Ascension of [Feb. 



From the measurements I have taken, which very nearly agree 

 with those Mr. W. Phillips was so kind as to take upon a small 

 crystal I gave him of this substance, I am led to take for the pri- 

 mitive form, an oblique rhombic prism in which the two lateral 

 planes are inclined to each other at an angle of 70° 40', the base 

 upon either of the lateral planes at an angle of 103°56 / , and in 

 which the ratio of one side of the base to the lateral edge is 

 nearly 100 to 97. 



The other incidences are, 



(m,fc) = 125° 20) (p, c") = 121° 30' (p, ft 3 ) = 95° 40' 

 (p, A 1 ) = 114° 55) (p, a*) = 99° 41' (m, b 5 ) = 160° 24' 



These crystals are accompanied by large green opaque crystals 

 of scapolite, lamellary black hornblend, and flesh-coloured lami- 

 nary carbonate of lime. The specimen comes from the mine 

 Neskiel, near Arendal, in Norway. 



Article XVIII. 



Corrections in Right Ascension of 37 Stars of the Greenwich 

 Catalogue, together with an Inquiry hoic far it would he advi- 

 sable that the Daily Corrections in R.A. and North Polar Dist- 

 ance of the 46 Zero Stai-s should be computed Annually at the 

 Public Expence. By James South, FRS. 



(Concluded from p. 45.) 



Having in the former part of this paper asserted, that the 

 corrections in right ascension of the principal fixed stars, can- 

 not be obtained piece-meal without considerable trouble, and 

 occasional error, we will now see how far the procuring correc- 

 tions in polar distance, is subject to similar inconveniences ; in 

 doing this, we will accompany the practical astronomer, first 

 into the observatory, secondly into the computing room. 



The instrument commonly employed for determining polar 

 distances, is a telescope attached firmly to a graduated circle, 

 and by means of micrometer microscopes placed opposite various 

 parts of its divided limb, the errors of eccentricity, division, and 

 partial expansion, are either destroyed, or rendered almost insen- 

 sible. The observations are generally conducted in the meri- 

 dian, and are extremely simple, consisting only in the accurate 

 bisection of the star, by the horizontal wire, situate in the focus 

 of the object glass ; this done, the different microscopes are read 

 off, and the mean gives the observed polar distance of the 

 star. 



To this, however, corrections must be applied ; generally for 



