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 effect numerically. 
The most important part of their observations were made 
on nebule ; 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 ;f 
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, 1845. 
+ Nos. 538, 739, 777, 844, 845, 854, 1797, 1833, 1907, 1929. 
} Nos.564, 706, 711,743, 748, 749, 805, 843, 846, 1146, 1385, 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. 
