126 



small, of a cluster such as 46 Messier (which is not common), 

 and a large planetary nebula (which is very rare) coinciding ; 

 and if we combine with this the probability of a round cavity 

 through one being exactly the size of, and in a line with the 

 other, that probability will be evanescent. It seems, there- 

 fore, necessary to conclude, that both are parts of the same 

 system, and possibly more examples of the kind may be 

 found. 



Two other clusters, 37 and 50 of Messier, besides their 

 own marvellous beauty, interested Dr. Robinson on another 

 ground ; they are in the Milky Way, and, therefore, are seen 

 on its stars, and at a place where its depth is nearly a maxi- 

 mum. Now, these stars were all of notable size and bright- 

 ness, so that the telescope evidently penetrated far beyond 

 their enter or limit. This seems to require a change in some 

 of the reasonings in Struve's admirable Etudes d' Astronomic 

 Stellaire. The author, among other curious matter, by apply- 

 ing the theory of probabilities to the numbers of stars of each 

 magnitude in Argelander's Catalogue, and Sir W. Herschel's 

 Star gauges, and by assuming that all stars are nearly equal, 

 and that the Milky Way is unfathomable by telescopes in its 

 greatest extension, finds this result, that the distance of the 

 sixth magnitude is about seven times that of the first, and that 

 the smallest stars visible in the eighteen-inch reflector of 

 Herschel are 251 times as remote as the sixth magnitude. 

 But this telescope should show stars at three times that dis- 

 tance, and hence he infers that the " heavenly space" is not 

 perfectly transparent. It appears to Dr. Robinson that the 

 last of these assumptions is inconsistent with the above-men- 

 tioned observation ; and that the other is equally at variance 

 with the arrangement so often referred to, in which the central 

 stars are much larger than the exterior. It may also be added, 

 that the penetrathig power of a telescope does not depend on 

 its light alone, for every one knows that a high magnifying 

 power shows small stars, which are invisible in the same tele- 



