472 Intelligence and Miscellaneous Articles. 



the Academy if it had beea anything but the immediate deduction 

 from facts and laws admitted by every one. 



Sir John Herschel, as far as I am aware, is the only person who 

 has indicated how weak was the absorption of light in traversing 

 comets, although nearly all other astronomers have perceived that 

 the nebulosity of comets did not sensibly weaken the light of the 

 smallest stars when these were seen through their tails, or even 

 through their nuclei. The following are the numbers relating to 

 the fourth comet of 1825, called the Great comet of the Bull (Hind's 

 Catalogue), discovered by Pons, at Marseilles, on the 15th of July. 

 It passed at the perihelium on the 10th of December; and at a mo- 

 ment when it could not yet have become hollow, namely the 15th of 

 August, Pons ascertained that a star of the fifth magnitude, which 

 was seen across its centre, had undergone no sensible diminution of 

 brilliancy. 



From this we may conclude that the star had not lost half a mag- 

 nitude, that is to say, a fifth of its light (if we admit, with Messrs. John- 

 son and Pogson, that a star diminishes one magnitude when its light 

 is reduced to fths of what it was originally) . The star consequently 

 retained at least f ths of its original brilliancy. 



We know also that the light of the stars in passing even perpen- 

 dicularly through the atmosphere, loses more than ^th of its inten- 

 sity, and is not more than f ths of what it was at its entrance into 

 the upper strata of the air. Taking 8 kilometres as the thickness of 

 the whole atmospheric stratum reduced to the density possessed by 

 this fluid at the surface of the earth, a single passage would reduce 

 the light to |ths of its original intensity ; two similar passages would 

 reduce it to |ths of |ths, or 'J^ths ; and lastly, a passage through a 

 space a thousand times greater would reduce the intensity to the 

 number f ths raised to the thousandth power. Now this is precisely 

 the passage made on the 15th of August 1825, by the light of the 

 star across the comet, the nucleus of which was more than 800 kilo- 

 metres in diameter. This number fths raised to the thousandth 

 power is a fraction having unity for its numerator, and for its deno- 

 minator a number of 5 figures followed by 120 cyphers. 



To assimilate the comet to dilated atmospheric air, we must there- 

 fore take air so rare that its density, multiplied by the above im- 

 mense number, would only be equal to the fraction f. Let x be this 

 hypothetical density, we should have 



We get for x a fraction having unity for its numerator, and for its 

 denominator a number superior to unity followed by 125 cyphers. 



When Herschel, in his last work on astronomy, spoke of a/ew 

 ounces as the mass of the tail of a comet, he found nearly as many 

 disbelievers as readers. Nevertheless his calculation is exaggerated 

 in comparison with the preceding determination. 



But how are comets visible ? I shall examine this question in ac- 

 cordance with my theory of the light which is disseminated by small 

 corpuscles forming a non-continuous medium. — Comptes Rendus, 

 May 4, 1857, p. 885. 



