180 The Rev. Dk. Robinson on the Constant of Refraction. 



disturbed by wind, or variations in the hygrometric state of the air. And it 

 must be remembered, that at least three-fourths of the entire refraction are pro- 

 duced in the region which is thus affected ; and that in observation we find 

 differences of 15 or 20 seconds in the same star, when the thermometer, barome- 

 ter, and hygrometer of the observatory shew no change. 



It appears to me, therefore, vain to expect an a priori solution of the problem 

 of astronomical refraction, and that it will always be necessary to reform by 

 observation whatever tables may be proposed to us. The tables of Bessel or 

 Ivory — (if the refractive and thermometrical constants of the latter were cor- 

 rected, I should prefer them) — are sufficiently exact for this purpose in the 

 observatories of Europe.* Down to 74° zenith distance, it is known, that the 

 law of density has no sensible effect on the refraction ; and in ordinary cases 

 this is sufficient for the astronomer, who seldom observes so near the horizon, 

 because there the fluctuations of a star are so great, that a great number of 

 observations are necessary to give even moderate precision. But he must occa- 

 sionally observe, under such circumstances, comets and planets ; and, besides, it is 

 necessary for an accurate determination of the principal constant, that he should 

 go as far from the zenith as is possible, without risking the certainty of his 

 correction. In my latitude, at 74° zen. distance, an error in the constant is 

 only doubled ; and the average discordance of observation will be near a second ; 

 so that were we limited to the use of stars above this altitude, it would be almost 



account it is stated, that the thermometer was steady at 30-75 cent. As light clouds existed far 

 above the balloon there must have been an evolution of heat from their formation. Still it is to be 

 wished that the experiment were repeated. 



* In the Arctic regions all the tables fail completely. I give a couple of instances from the 

 Appendix to Parry, already noticed, p. 209. They are Nos. 25 and 29. The first gives from 108 



observations, the refraction = 665".9 at zen. dist. 84°.13', 82, Bar. 29.79, A. T. -|- 45, Ext. T 



35°.9. After correcting for latitude, Bessel's refraction is 18" .72 in defect. Ivory's 13" .27, and 

 mine 20''.7I. Again, 32 observations give refraction =r 342" .5 at 79°40'. 61, bar. 29.86, A. T. 

 + 45°, E. T. - 260.7. Here Bessel's is 40".31 in excess, Ivory 31".66, and mine 22".78. It seems to 

 follow from these and similar instances, that in such extreme cases the arrangement of the atmos- 

 phere must be regulated by very different laws from those that prevail in more temperate latitudes ; 

 and it seems equally obvious, that its influence on refraction commences much nearer the zenith. 

 It is my intention to recur to these Arctic observations in a subsequent communication on the lower 

 refractions. 



