64 



but when the zenith distance is greater than 80°, the author con- 

 siders the accuracy of the French table questionable, both on account 

 of the hypothetical law of the densities, and because the quantity 

 assumed for the horizontal refraction is uncertain. 



After giving a few examples, illustrative of the use of the new 

 table, the author inquires how far the refractions are likely to be 

 affected by the term which it was found necessary to leave out, be- 

 cause the present state of our knowledge of the phenomena of the 

 atmosphere made it impossible to determine the coefficient by which 

 it is multiplied. For this purpose, the variable part of that term 

 has been comi^uted for every half degree, from 85° to 88°, and the 

 results are exhibited in a table. From this it appears, that this co- 

 efficient, although considerably less than that of the preceding term, 

 may still have some influence on the refractions at very low alti- 

 tudes. The mean refraction in Bessel's table, and in the new table, 

 can hardly be supposed to differ 2" from the true quantity, which 

 would limit the coefficient in question to be less than one- tenth. It 

 is a matter of some importance to obtain a near value of this coeffi- 

 cient ; and it is probable that this can be accomplished in no other 

 way, than by searching out such values of the two coefficients as will 

 best represent many good observed refractions at altitudes less than 

 5°. If such values were found, our knowledge of the decrease of 

 heat in ascending in the atmosphere would be improved, and the 

 measurement of heights by the barometer would be made more per- 

 fect. 



At the end of the paper is given a table of mean refractions for 

 the temperature 50° Fahr. and barometric pressure 30 inches, at 

 every degree from 0° to 70° zenith distance, and at every 10' from 

 70° to the horizon ; and tables of the corrections requisite for va- 

 riations of the thermometer and barometer are subjoined. 



May 31, 1838. 



DAVIES GILBERT, Esq., V.P., in the Chair. 



The Rev. John Hymers was duly elected a Fellow of the So- 

 ciety. 



A paper was read, entitled, " Remarks on the Theory of the Di- 

 spersion of Light, as connected with Polarization." By the Rev. Baden 

 Powell, M.A., F.R.S., Savilian Professor of Geometry in the Uni- 

 versity of Oxford. 



The present paper is a sequel to those already presented by the 

 author to the Royal Society, in which he had instituted a compari- 

 son of the observations of the refractive indices for the standard rays 

 of light in various media, with the results calculated from theoretical 

 formulse, deduced from the most improved views of the undulatory 

 hypothesis ; the cases discussed including the greatest range of 

 data as yet furnished by experiment. The comparison exhibited an 

 accordance sufficient to warrant the conclusion that the theory af- 



