Clusters and Nebuloe. 465 



[45.] 2 5 N(eb.) Gen. Cat. 4234. E.A. xvih. 39m. D.K 

 24° 4'. To find this we must identify two guide- stars, j3 and 

 e Herculis. The former will be recognized by the directions 

 in Int. Obs., ii. 56 (under k Here.) • the latter from those for 

 f, in Int. Obs., vi. 1 1 7, since e is the next 3 mag. star / f, a 

 little s. About one-third of the distance from /3 to e, but con- 

 siderably s of the joining line, we mnst sweep — and in this 

 case, to our annoyance, not with the finder, in which the 

 object would not be distinguished from a small star, bnt with 

 the telescope itself, and not with a very low power, for the 

 same reason. With 3^ in. I found it better seen with 144 

 than 80 ; with 5| in. it was not quite stellar even with a comet- 

 finding power of 30, though it would have been altogether so 

 with a lesser aperture. 



The reason of this disadvantage in smaller instruments is 

 worthy of notice. The apparent telescopic diameter of a star, 

 or its spurious disc, is enlarged, from the nndulatory nature of 

 light, and the interference occasioned by our insulating a por- 

 tion of it by the telescope, in proportion to the diminution of 

 the aperture. Old Hevel, who claims to have been the first to- 

 notice this, employed his discovery in an amusing way. The 

 fact had been previously remarked that the stars had no 

 visible circular discs in the telescope, but appeared as radiating 

 and sparkling points. Galileo and others had rightly divined 

 the cause — the excessive distance, on which magnifying power 

 would produce no sensible effect ; and the sagacious Kepler 

 had gone a step further in observing that the stars appeared 

 smaller in proportion to the excellence of the telescope. But 

 Hevel rejoiced in the discovery that by the application in front 

 of the object-glass of a diaphragm of the size of a large pea ! 

 (which, he confesses, destroyed the definition of the moon) he 

 could see them as circular discs of a sensible magnitude, vary- 

 ing according to the brightness of the star ; which he says 

 anyone else might do who knew how to use his telescope aright* 

 Little did the worthy Biirgermeister imagine that he was 

 intentionally producing*, in 1047, what it is now the object of 

 every observer to get rid of as much as possible. In pro- 

 portion to his contracted aperture he was increasing the effect 

 of interference, and in doing what he could to make the stars 

 look like planets, was making them look especially unlike 

 themselves. Now this enlarging effect will be more con- 

 spicuous on stars than other objects, because of their more vivid' 

 light : the luminous border, so to speak, added by interference > 

 being much less intense, as Airy has shown, than the interior 

 of the disc, may be strongly perceptible with the native radiance 

 of the stars, when with reflected lunar or planetary light it 

 impairs definition in a less obvious degree. (And thus by the* 



VOL. XI. — NO. VI. H II 



