362 ANNUAL OF SCIENTIFIC DISCOVERT. 



movements of the planetary bodies. "We shall see that from the effects ob- 

 served, combined with the law of optics, we may deduce the conclusion, 

 that the direct shock of one of these bodies could not cause the penetration 

 of the infinitely rarefied matter of which they arc composed, even into our 

 atmosphere. 



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

 tude, 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 Hcrschel, Piazzi, 

 Bessel and Struvc'. In most instances, says Mr. Hind, there is not the least 

 perceptible diminution in the brilliancy of the star. 



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 Struvc saw a star of the eleventh magnitude 

 through its central part, without noticing a 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, docs net sensi- 

 bly weaken the light of a star in front of which it forms a luminous current, 

 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 sensible. We may, therefore, assume, that at 

 the utmost the brilliancy of the comet equalled a sixtieth part of the light 

 of the star. Thus, l>y this hypothecs, 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 is to say, GGOO times 

 more luminous than it was, it would then have been sixty times more lumin- 

 ous than the star, and in its turn would have made the latter disappear by 

 the superiority of its lustre. 



The conclusion from this is, that it would have been necessary to illumine 

 the comctary substance more than 3GOO times more than it Avas illumined by 

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

 magnitude. 



We mny assume that the light of the moon causes the disappearance of 

 all the stars below the fourth magnitude; thus the atmosphere illumined by 

 the full moon acquires sufficient luminosity to render stars of the fifth and 

 all lower magnitudes invisible. Between the fifih and tho eleventh magni- 

 tudes there arc six orders of magnitude, and according to the fractional 

 relations of these different orders we mav admit that a star which is a single 



J 



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

 than the latter. A star of the fifth magnitude is 250 times more brilliant 

 than a star of the eleventh magnitude. Thus the illumination of the atmos- 

 phere by the moon is much more intense than the illumination of the comct- 

 ary substance by the sun itself, since it would bo necessary to render tho 

 cornet SGOO times more luminous to enable it to extinguish a star of the 



