36 REPORT — 1872. 



Light. 

 JRefraction and Solar Sjjots. B>/ J. H. Brown. 



On the Action of Quartz on Ultra- Violet Bays. By Professor Cbotjilebois. 



On Tales Phosphorescent by Friction. By Professor Ceoullebois. 



On Focal Lines. By Professor J. J). Everett, D.C.L. 



On a Difficulty in the Theory of Aberration. 

 By Professor J. D. Everett, D.C.L. 



On Mirage. By Professor J. D. Everett, D.C.L. 



On Astronomical Refraction, By George Forbes. 



The errors of tlie refraction tables are best shown by noticing the variations in 

 the North Polar Distances of stars observed with the Greenwich Transit-circle ae 

 determined by observations of different nights. They are sometimes very con- 

 siderable. Humidity is doubtless one of the most important points to be attended 

 to. But this correction is difficult to appl}', for its value is at present unknown. 

 The author wished to point out a minor correction, which, however, becomes im- 

 portant in some cases, which can be easily determined, and which, so far as he 

 Knows, has not been hitherto alluded to. This is the effect due to a difference in 

 the heiyht of the atmosphere at adjacent stations on the meridian (if a meridian 

 instrument be used), as shown by the barometer. The superabundant air will act 

 as a prism of air, and may possibly introduce sensible errors. 



The theory of correcting for this is as follows : — 



Li the difterenrtial eq^uation to the path of a ray, viz. 



dr=-!^ tan i, 



i is the inclination of the ray to the normal to a surface of equal density (the sur- 

 faces of equal density being supposed to be concentric). J3ut in the case considered, 

 where the barometer varies at adjacent stations, tlie surface of the earth is not a 

 surface of equal density, but is inclined to it, so that the sections of the surface of 

 equal density and the surface of the earth, by a plane in the meridian, include an 

 angle a, which can be easily tabulated for different values of the barometric differ- 

 ences. Hence, in using Bessel's refraction tables, where the argument is the angle 

 i, we ought to use, not the observed zenith-distance Z, but the angle Z+a. 



By comparing good observations at stations five miles apart, the author found that 

 the barometers sometimes differed by O'OIO inch. The effect of this difference on the 

 places of stars is as follows : — 



Tor stars whose zenith-distance is 60 the coiTection is about 0-2 

 >} !> 80 „ „ 1-0 



f> }> 85 „ „ 3-6 



90 „ „ 200 



These effects then are very sensible. Nor is it likely that the barometer obser- 

 vations compared are exceptional. 



The existence of this source of error was clearly detected in the Greenwich 

 observations, by comparing them with observations of the barometer at adjacent 

 stations. Every test that has been apj^lied confirms the opinion that, by the appli- 

 cation of tliis correction, a considerable increase of accuracy would be obtained in 

 stars of great zenith-distance. 



