November, 1912. 



KNOWLEDGE. 



433to438)—M. 11730 Ursae Majoris, H.V. 43 Ursae 

 Majoris. M. 100 Comae Berenices, M. 99 of the same 



constellation. M. dl \'irLMnis. anil M. SN Comae 



1-I(,L'KK 4J>1. 



The .\ni)ular N'ebuLi in Lyra. 



Trom a photo,L;rapli taken at Mtniiit Wilson Observatory. September 



17tli. 1909. by G. \V. Ritcliey, Exposure thirty minutes. 



Berenices. It is only with large instruments and 

 the highest applications of the photographic art 

 that the beauty and wonder of these objects becomes 

 to some extent manifest. With smaller instrumental 

 means but little information can be gathered. The 

 bulk of the objects given in Messier's catalogue have 

 proved to be remote or highly condensed star 

 clusters ; only a few of them appear to be trur 

 nebulae. It was on account of this resolution b\ 

 the application of more powerful optical appliances 

 that the idea was once held that the only diftcrencu 

 between these objects was mereh' a matter of greater 

 or less distance, the nearer clusters being show n to 

 be such by small telescopes, the more remote ones 

 proving intractable. The great telescopes of the 

 Herschels and Lord Rosse succeeded in bringing so 

 many of these objects hitherto unresolved into tht' 

 resolvable class and it was at one time even thought 

 that the great nebula of Orion itself, most wonderful 

 of them all, showed signs of resolvabilit\'. On 

 philosophic rather than strictly scientific grounds 

 the idea had been entertained of the existence of a 

 specific form of substance, " nebulous matter," of 

 extreme tenuity and transparency, with feeble 

 luminosity, differing from the solid, liquid, and 

 gaseous conditions with which we are familiar 

 on our planet. From this matter it was supposed 

 all other matter had arisen by slow condensa- 

 tion, and the irresolvable nebulae were formed 

 of this as yet uncondensed material. These 

 views were supported b\- Halley, to whom 



science owes so much both in the way of dis- 

 covery and suggestion: he supposed "the light 

 of the nebulae to be occasioned by a lucid medium 

 diffused through the ether and shining by its own 

 light." Up till quite recently, however, it has been 

 generallv considered that all these bodies were con- 

 tinuous masses in e(iuilibrium or revolving slowly. 

 On the Laplacian nebular hypothesis it was supposed 

 that such a mass bv slow contraction gradually left 

 behind rings of matter, which collected into separate 

 globes and formed the planets. The nebula of 

 .\ndromeda was thought by Sir G. Darwin and 

 others to be an actual example of this process going 

 on before our eyes. But when we consider the 

 enormous size and consequent!}' small density of the 

 nebulae in general, we shall understand the difficulty, 

 nay, the impossibilit}', of any such condition of 

 aftairs. The late Mr. Proctor, editor of " Know- 

 i.KOGE," long ago pointed out other objections. 

 " The nebulae we see have, it seems, a greater analogy 

 with the solar corona than with the fiery con- 

 densing mists conceived of by Laplace" ("Old 

 and New .\stronomv," >; 1445). In another place 

 he says: "Whenever enquiry is made into the 

 Laplacian nebular h)-i)othesis, that will be even more 

 decisively rejected." The views of so careful a 

 w riter, who took nothing for granted on the authority 

 of others, are worthy of our most scru[)ulous con- 

 sideration. Many of the nebulae have an apj)arent 

 diameter greatly exceeding that of the Sun or Moon, 

 sometimes extending over the greater part of a 

 constellation : but their luminosity is so small that, 

 as we have said, it is only by the highest optical 

 power that the full extent is realised. On the other 

 hand, stars evidently intimately connected therewith 



Figure 440. 



The Cluster M. 13 in Hercules. 



From a pbotoyrapli taken at Lick Observatory on July 13th, 1899, 

 with a Crossley Reflector. Two hours exposure. 



