A stronomy — Optics. 3^5 



Connoiss. des Terns, 1827, p. 223, 224. 



4. Depression of the Horizon at Sea. — In an interesting notice by M. 

 Arago on this subject, he compares the differences between the calculated 

 and observed depression with the differences between the temperature of 

 the air and the sea as observed at the same time. When the calculated 

 depression is greater, the errors are called positive, and when they are less, 

 negative. M. Arago found that the error of the computed depression will 

 he positive in a climate where the temperature of the air exceeds that of 

 the sea, but that the negative errors are observed indiscriminately in all re- 

 lative temperatures of the air and the sea. Connoiss. des Terns, 1827, p. 

 319. 



OPTICS. 



5. Refractive Power of Dry and Humid Air. — M. Arago has found by 

 a particular method, that the refractive power of humid air differs a very 

 little from that of dry air, the elastic force of each being the same- Con- 

 noiss. des Terns, 1827, p. 320- 



6. Polarisation of Light from Solid or Fluid incandescent Bodies.— M. 

 Arago has observed, that the rays which issue from solid or fluid incandes- 

 cent bodies are partly polarised by refraction, when they form with the 

 surface of emergence an angle of a small number of degrees. The light of 

 combustible gases presented no traces of polarisation. Hence M. Arago 

 concludes, that a considerable portion of the light of incandescent bodies is 

 formed in their interior, and at depths which he has not yet completely 

 determined. Ann. de Chim. torn, xxvii. p. 89. 



7. Optical Phenomena observed by M. Ruppell. — In observing the 

 eclipses of the stars by the moon, near the ruins of Solib in Upper Egypt, 

 M. Ruppell had, on the 4th of June, directed his large telescope to the ob- 

 scure limb of the moon. Close to it he observed a star of the 5th magni- 

 tude, which was about to be eclipsed by the moon. When it was near the 

 limb, and on the point of disappearing, he observed, to his great surprise, 

 that the star of the 5th magnitude was divided into two smaller ones of the 

 8th magnitude, which he saw with extraordinary distinctness. A few se- 

 conds afterwards they successively immerged behind the moon's limb. M. 

 Ruppell asks, was this distinctness of vision produced by the atmosphere 

 of the moon ? Baron Zach explains this effect by saying, that M. Ruppell 

 saw the star better, and consequently double, (for we presume it was real- 

 ly a double one,) in consequence of his looking longer at it; but this can 

 never be considered as an explanation of the extraordinary dittinctness of 



