125 



having ascertained the parts where the stars were most dis- 

 tinct, he was able to see them in the three feet with certainty ; 

 though in former years he had repeatedly scrutinized it for 

 this very purpose in vain.* 



Two remarkable exceptions to the general plan of nebular 

 systems are afforded by 64 Messier, and h 464. In general the 

 centre is occupied by a cluster of comparatively large stars, 

 round which the others are grouped. But in the first of these 

 (Herschel's fig. 27) there is a central vacancy looking abso- 

 lutely black by contrast with the surrounding mass of stars. 

 At its south and preceding edge are disposed, rather irregu- 

 larly, a knot of about 100 larger stars, of which it is scarcely 

 possible to doubt that they had once formed the usual globular 

 cluster in the vacancy^ and had been in some way displaced 

 from it. The second is a fine planetary nebula in the splen- 

 did cluster 46 Messier. The stars of the latter are large and 

 very brilliant, so that probably it is not very remote; but the 

 other is a round disc, entirely composed of minute blue stars, 

 without any condensation in the middle ; and the singularity 

 is, that it is not encroached on by the stars of 46 Messier. 

 One very large one is near its edge ; but evidently it would not 

 be possible to describe a circle of equal diameter in any other 

 part without including several. Are we to suppose that this 

 is a case of mere optical connexion ? The probability is very 



* A recent notice mentions that Mr. Bond, of Harvard University, in the 

 United States, has resolved parts of this nebula wilh a Munich achromatic 

 similar to that of Pulkova. The climate and lower latitude would assist him 

 in some degree ; but Dr. Robinson thinks his success must be in a great mea- 

 sure due to that precise knowledge of the phenomenon, and of the points 

 where it might be looked for, which is afforded by Dr. Mchol's work. He 

 perceived the fifth and sixth stars of the trapezium, but saw nothing of the 

 new pair. It must be remembered that, however sharply an achromatic may 

 define objects whose light is intense, its illuminating power is far inferior to 

 that of a large reflector. An object-glass of sixteen inches has not as mucli 

 light as a Ncwtoniun of twenty-one. 



