127 



states (Smith's Optics) that a piece of silver wire can be 

 seen on white paper, when it subtends 3'', a result depending 

 on the intensity of this metallic reflection. This would give 

 eighteen feet! Dr. Robinson finds that he can see the 

 spider lines of his circle without much contrast of light, when 

 they subtend to him 16". This gives ninety-seven feet ; 

 but it must be remembered that aperture influences visibility 

 as well as magnifying power, though we cannot as yet estimate 

 its efi"ect numerically. 



The most important part of their observations were made 

 on nebulae ; and, besides establishing completely the prodi- 

 gious superiority of this instrument over all yet constructed, 

 they have added some facts to our knowledge of these myste- 

 rious objects. A list of them was formed from the invaluable 

 catalogue of Sir John Herschel (Phil. Trans. 1833), compris- 

 ing such as, from brightness or any other peculiarity, seemed 

 deserving of notice ; of which forty were examined by Dr. Ro- 

 binson and also by Sir James South, except some which the 

 latter lost while making the transit observations required for 

 the meridian line.* They may be separated into three classes ; 

 those which are round and of nearly uniform brightness;! 

 those which are round, but appear to have one or more nu- 

 clei ;$ and those which are extended in one direction, some- 

 times so much as to become long stripes or rays.§ Of the 

 first class, all that were examined are easily resolved, even 

 with a triple eye-piece of wide field and power 360, used for 

 finding the objects. In 854 the stars were seen through haze ; 

 in 1929 during twilight; and 1833 was noted as " consisting 



* Sir James South published an interesting and instructive notice of this 

 telescope in the Times, April 16, 1843. 



t Nos. 538, 739, 777, 844, 843, 854, 1797, 1833, 1907, 1929. 



X Nos.564, 706, 711,743, 748, 749, 805, 843, 846, 1146, 1383, 1456, 1622, 

 1881. 



§ Nos. 536, 604, 668, 791, 792, 810, 859, 1066, 1132, 1148, 1352, 1357, 

 1368, 1466, 1926. The numbers and the figures cited in the text are those 

 of Sir John Herschel's catalogue. 



