xli 



Regulus was passing ; that star was shown as a round point of intolerable 

 light ; but as the night wore, the definition declined till e Bootes came, 

 which it only showed as two coloured flares, and Lord Hosse gave up further 

 observing as useless. On October 20, 1848, under similar cicmnstances 

 of uncovering the speculum, the blue star of y Andromedae was seen with 

 1500 divided a full diameter of the larger component, the colours blue and 

 purple. The previous evening, after a longer exposure, the star looked 

 elongated, and the division appeared only by glimpses. For this there 

 seem but two possible remedies, to keep the speculum as near the tempe- 

 rature of the air as possible, and to make the tube of open work, so that 

 the warmed air may escape as soon as possible. The first of these was 

 not easily practicable with so huge a block of metal, though he thought it 

 could be effected ; but the other, which was many years ago suggested by 

 Sir John Herschel, he tried on the three-feet, and with such success that 

 he resolved to apply a similar tube to the six-feet when it should re- 

 quire renewal. This will undoubtedly extend its good definition through 

 a greater portion of the night, and make it more potent for the resolu- 

 tion of nebulae. But, even under average atmospheric conditions, its power 

 is astonishing. No achromatic in Britain gives any adequate notion of 

 the way in which it shows the moon, close contrasted double stars, or 

 clusters like 13 Messier. Its chief employment, however, has been on 

 nebulse, and the results which they have afforded are very remarkable. 

 That it resolved a multitude of them which resisted all other telescopes 

 was a matter of course ; but it also revealed in many of these strange bodies 

 the existence of forms and forces completely at variance with our previous 

 conceptions of celestial mechanics. Many of the observations are given in 

 our 'Transactions' for 1850 and 1861 ; and it may suffice to indicate a 

 few of their results. The most remarkable is the spiral arrangement, 

 which seems to prevail very commonly in nebulse, and occasionally in 

 clusters. Sometimes the spirals are as regular as the fire- curves of a pyro- 

 technic wheel ; sometimes, when seen obliquely, like the section of a 

 snail-shell ; sometimes almost foreshortened into a ring ; occasionally there 

 is a double system ; and in one or two instances the two sets turn in dif- 

 ferent directions. The dynamical condition which is implied by these 

 appearances cannot be explained by any cosmical forces with which we are 

 acquainted, and it is an object of the highest interest to science that they 

 should be carefully watched for any signs of change. 



It has also been ascertained that a remarkable class of nebulae, called 

 Planetary from appearing like uniform circular or elliptic disks, have no 

 existence ; they are shown to be systems of rings, sometimes very compli- 

 cated, so that annular nebulae, of which only two were previously known 

 in our hemisphere, are by no means uncommon. And thirdly, some in- 

 formation has been gained as to nebulous stars. These rare objects differ 

 from ordinary stars in having a round atmosphere of some extent, and 

 faintly luminous. This is not continuous, as had been supposed ; some- 



