PHILOSOPHICAL TRANSACTIONS. 



IX. The Bakerian Lecture. — On the Theory of the Astronomical Refractions. By 

 James Ivory, K.H. M.A. F.R.S. L. ^ E., Imtit. Reg. Sc. Paris, Corresp. et Reg. 

 Sc. Gottin. Corresp. 



Received April 26,— Read May 3, 1838. 



J. HE apparent displacement of the stars caused by the inflection of light in its pas- 

 sage through the atmosphere, is treated by the astronomer like most other irregula- 

 rities which he has occasion to consider. A set of mean quantities is first provided ; 

 and the occasional deviations of the true places from the mean are ascertained and 

 corrected according to the state of the air, as indicated by the meteorological instru- 

 ments. The subject of the astronomical refractions is thus resolved into two parts 

 very distinct from one another ; the first embracing the mean refractions, which are an 

 unchangeable set of numbers, at least at every particular observatory ; the second re- 

 lating to the temporary variations occasioned by the fluctuations which are inces- 

 santly taking place in the condition of the atmosphere. It is the first of these two 

 questions chiefly, or that regarding the mean refractions, of which it is proposed to 

 treat in this paper. 



In order to form a just notion of the mean refractions, we may suppose that some 

 particular star is selected, and assiduously observed for a course of time so consider- 

 able as to comprehend every possible change in the condition of the' atmosphere ; all 

 these observed places being severally reduced to some assumed state of the thermo- 

 meter and barometer, and being combined so as to eliminate occasional irregularities, 

 will determine the mean refraction of the star. In this procedure it is supposed, what 

 experience confirms, that the result will ultimately be the same for the same altitude 

 above the horizon, provided the observations are numerous enough, and extend over a 

 sufficient length of time. We may instance the star a Lyrse observed by Dr. Brinkley ; 

 his observations are forty-four in number, extending over five years ; and the greatest 

 deviation of single observations from the mean quantity may be stated at i 20". The 

 supplementary Table, extending from 85° to 89°^ of zenith distance, published in 

 Bessel's Tabulce Regiomontance, is one of mean refractions calculated from many 



MDCCCXXXVIII. z 



