542 



combinations, that Sirius is as much brighter than Canopus, 

 as a Centauri is brighter than Achernar. 1 hope to be able 

 to pursue these investigations. 



NOTE D. 



The following is an extract of the observations on the mi- 

 rage, made in 1799 and 1800, during my stay at Cumana, 

 as I noted them in my Astronomical Journal. I could not 

 at that time be acquainted with the theory of Mr. Monge, 

 and the experiments of Messrs. Brandes, Wollaston, and 

 Tralles. Those of the celebrated English philosopher were 

 made at the same period as mine. Mr. Vince contented 

 himself with following with his telescope the phenomena of 

 suspension, without determining the magnitude of the images 

 and the depression of the horizon of the sea. These deter- 

 minations were also wanting in the researches of Mr. Buesch 

 at Hamburgh (Tractatus duo optici argumenti) ; and of the 

 Abbe Gruber (Ueber Stralenbrechung und Abprallung des 

 Lichts, 1793) . Though in 1800 I had only a vague idea of 

 the different circumstances, which modify the mirage, I did 

 not neglect to measure the angles of depression of the hori- 

 zon, the breadth of the interval between the horizon and the 

 suspended object, the temperature of the sand, above the 

 surface of which the rays of light passed, and that of the air 

 and of the water. I examined the influence of the figure of 

 the small islands on their more or less complete suspension ; 

 the circumstances where the suspension takes place without 

 double images ; and at length the changes, which the rising 

 or setting sun occasions in the appearances of these extraor- 

 dinary refractions. (See vol. ii, p. 196 3 and chap, xi, p. 

 358 of the present vol. ) 



" Cumana, lat. 10° 27 1 52". Terrace of the house of Don 

 Pasquel Martinez, which I have inhabited since my return 



