302 Intelligence and Miscellaneous Articles. 



bodies could not cause the penetration of the infinitely rarefied matter 

 of which they are composed, even into our atmosphere. 



It is a well-ascertained fact that stars of the tenth and eleventh 

 magnitude, and even lower ones, have been seen through the central 

 part of comets, without any sensible loss of brilliancy. Amongst the 

 observers who have frequently proved this optical fact, we find the 

 names of Herschel, Piazzi, Bessel, and Struve. In most cases, says 

 Mr. Hind, there is not the least perceptible diminution in the bril- 

 liancy of the star. As to the pretended phases of the cometary 

 nuclei, the direction of the line of the horns was incompatible with 

 the hypothesis of the illumination of an opake nucleus, and the 

 modern representations of the appearance of comets readily explain 

 the error of those who have assumed opake nuclei. I shall take as 

 an example the well-known comet of Encke, which is sometimes 

 visible to the naked eye, and generally presents a rounded mass. 

 In 1828 it formed a regular globe of about 500,000 kilometres in 

 diameter, with no distinct nucleus ; and M. Struve saw a star of the 

 eleventh magnitude through its centi^al part, without noticing any 

 diminution of brilliancy. In an observation of M. Valz, on the other 

 hand, a star of the seventh magnitude almost entirely effaced the 

 brightness of a brilliant comet. Let us start from these observed 

 facts. 



Since the interposition of a comet illuminated by the sun does not 

 sensibly weaken the light of a star in front of which it forms a lumi- 

 nous curtain, it follows that the brilliancy of the comet is not a 

 sixtieth part of that of the star, for otherwise the interposition of a 

 light equal to a sixtieth part of that of the star would have been sen- 

 sible. We may therefore assume, that at the utmost the brilliancy 

 of the comet equalled a sixtieth part of the light of the star. Thus, 

 by this hypothesis, if the comet were rendered sixty times more 

 luminous, it would have a lustre equal to that of the star ; and if it 

 had been rendered sixty times sixty times, that i- to say three thou- 

 sand six hundred times, more luminous than it was, it would then 

 have been sixty times more luminous than the star, and in its turn 

 would have made the latter disappear by the superiority of its lustre. 

 The conclusion from this is therefore that it would have been 

 necessary to illuminate the cometary substance more than three 

 thousand six hundred times more than it was then illuminated by 

 the sun, to enable it to cause the disappearance of a star of the eleventh 

 magnitude. 



We may assume that the lightof the moon causes the disappearance 

 of all the stars below the fourth magnitude ; thus the atmosphere 

 illuminated by the full moon acquires sufiicient luminosity to render 

 stars of the fifth, and all lower magnitudes invisible. 



Between the fifth magnitude and the eleventh, there are six orders 

 of magnitude, and according to the fractional relations of these dif- 

 ferent orders, we may admit that a star which is a single degree of 

 magnitude above another, is two and a half times more luminous 

 than the latter. In the publications of the Observatory of Oxford, 

 we may see a good compilation of that excellent astronomer, Mr. 



