452 The Achromatic Telescope. 



not met with any subsequent report of the action of the six-feet 

 mirrors upon it. Its common name is derived from the branch- 

 ing filaments, in which some resemblance may be fancied to the 

 claws of a crab ; but the student must be content to trace it 

 in the engravings which are often met with, as no ordinary 

 instrument will touch these appendages, and they were not 

 remarked even by Sir J. Herschel. With a small aperture it 

 will be found a feeble object, though very distinguishable from 

 its magnitude; with my h\ inches I thought I could see a 

 mottled character pointing to resolution, and its edges, being 

 known to be actually fringed, were suspected to look so. 



A short distance from £, p.. a little n, we shall find a pretty 

 little open double star. 



Addendum to No. 4, in our last number, p. 351. 



I have since ascertained that with the six-feet speculum of 

 the Earl of Eosse the brightest part of the great Nebula in the 

 Triangle has been found to exhibit that marvellous spiral 

 arrangement, to which we shall have to refer more fully here- 

 after. From a common centre spring four thick branches (with 

 a mere suspicion of a fifth), somewhat like the arms of a cross, 

 of unequal length, but each curved in the same direction. — Three 

 observations, 1849 ; at the last of which the whole nebula was 

 seen in " nocculi." No other portion of it seems to have any 

 definite arrangement. 



OCCULTATIONS. 



Jan. 15th, 7rPiscium, 6 mag., will be hidden (at Greenwich) 

 from 12h. 8m. till 12h. 44m. 19th, I Tauri, 5| mag., from 12h. 

 2m. till 12h. 38m. 20th, p^ 1 Orionis, 4£ mag., will disappear at 

 7h. 57m., and reappear at 9h. 13m. 24th, k Cancri, 5 mag., at 

 Ch. 30m. and 7h. 15m. respectively. 



THE ACHEOMAT1C TELESCOPE. 



The statistics of telescopic improvement and diffusion, if at- 

 tainable, would form a curious subject of inquiry. Many an 

 astronomer now living well remembers the day when an achro- 

 matic telescope of even 3 f inches aperture was the largest that 

 could be found as a portion of the optician's regular stock, and 

 was by no means a cheap article, while those of greater dimen- 

 sions were as scarce as they were costly. ^ Now, those of the 

 larger class are perhaps almost as numerous as the smaller ones 

 were not many years ago; and it is at the same time a very 

 gratifying consideration that this remarkable increase both in 

 quantity and size has not been attended with any deterioration 

 of quality, but rather the reverse. It may be doubted whether 

 the high character which was ascribed to some of the earlier 

 achromatics was other than comparative. They could not appear 





