Sir D. Brewster on the Law of Daily Temperature. 341 



brilliancy) is to render the bands more or less distinct without 

 changing their places. 



All these theoretical results are in absolute accordance with 

 Prof. Powell's observations. 



The fact, that the bands can sometimes be seen on only a 

 portion of the spectrum, is a result of this circumstance, that 

 the retarding plate cannot be placed close to the pupil of the 

 eye. A single trial will enable any one to perceive that the 

 further the plate is held from the eye, the smaller is the extent 

 of visible bands; and the injurious effect of this circumstance 

 is much greater when the pupil is small than when it is large ; 

 inasmuch as, with a very small pupil, the light which enters 

 the eye from extreme points of the spectrum may not have 

 touched the retarding plate at all. 



I will conclude with observing that this very remarkable 

 experiment, which long appeared inexplicable, and which at 

 two subsequent times has presented startling difficulties to the 

 theorist, seems destined to give one of the strongest confirma- 

 tions to the Undulatory Theory. 



I am, Gentlemen, 



Your obedient Servant, 

 Royal Observatory, Greenwich, G. B. AlRY. 



Oct. 16, 1846. 



LII. Observations on the Law of Daily Temperature. By 

 Sir David Brewster, K.H., D.C.L., F.R.S., and V.P.R.S. 

 Edin.* 



IT followed from the early researches of Humboldt, that the 

 mean temperature of the equatorial regions was nearly the 

 same under every meridian, but that in higher latitudes it de- 

 clined rapidly in the New World, and under the eastern me- 

 ridians of Asia, as shown in the following table: — 



But notwithstanding the inflexion of the isothermal lines, 

 as thus indicated, the Arctic and Antarctic poles were consi- 

 dered as the coldest points of the globe. Having found that 

 in the West of Europe the mean annual temperature varied 

 with the cosine of the latitude, and not, as had been believed, 



* The substance of the following article formed the concluding portion 

 of a review of Baron Humboldt's Asie Centrale, published in the North Bri- 

 tish Review for August 1846, vol. v. p. 492. 



t Mem. d'Arcaeil, torn. iii. p. 462, and. Edin. Phil. Journal, vol. iii. pp. 4, 

 256 ; vol. iv. pp. 23, 262,- and vol. v. p. 28. 



