Notices respecting Neiso Books. 383 



cuous of the remaining ones were determined from them hy trian- 

 gulation ; — the angles being measured with the position micrometer 

 of the twenty-foot reflector, and projected on the chart by means of 

 a protractor. From the stars thus laid down by triangulation, 

 others depending on eye-drafts were then inserted in the chart ; 

 and, finally, the differences in RA and NPD were read off from 

 the chart by means of diagonal scales, and the whole entered in 

 a catalogue. The uebulaj were then worked in upon the chart ac- 

 cording to the united evidence of all the drawings and diagrams. An 

 idea will be given of the immense labour bestowed on these drawings 

 by stating, for a few cases, the number of small stars within their 

 boundaries whose places were approximately determined. In the 

 monograph of the nebula marked h. 2941, the number of stars laid 

 down is 105 ; in h. 3722, the number is 186 ; in the cluster h. 3435, 

 110 ; in the nebula about fl Orionis, 150 ; and so on. 



We have no room to enter into details respecting the objects 

 figured in the six plates which belong to this chapter, and compres- 

 sion or abridgement is scarcely possible. We shall therefore content 

 ourselves with a brief allusion to the two principal ones : — the great 

 nebula in the sword-handle of Orion, and that about the star rj Argus. 

 The former of these objects was described by Sir John in 1824 (Mem. 

 R. Astron. Society, vol. ii.), and subsequently by Dr. Lamont of 

 Munich, as also by Rondoni, a Roman artist, and by the astronomer 

 De Vico. He observes, — 



" It may be supposed that in a situation so favourable for viewing 

 this magnificent object as the Cape, where it passes the meridian at 

 an altitude of 60°, with the additional advantage of a sky of perfect 

 purity, and of mirrors in a constant course of repolishing, I should 

 eagerly seize the opportunity to re-examine xay earlier delineation 

 of it, with a view to the detection of change, the correction of error, 

 and the observation of further particulars as to its form, extent and 

 structure which had escaped previous observation." The first glance at 

 the object, under these favourable circumstances, showed a multitude 

 of nebulous branches, convolutions, and other details of which he had 

 not before had the least suspicion. Great pains were accordingly 

 taken in the delineation of the figure here given. He observes that 

 although a comparison of it with the former representations might 

 convey an impression of great and rapid changes in the nebula, he is 

 far from participating in such an impression, and is disposed to think 

 that the disagreements in his own two drawings, though confessedly 

 great, are not more so than may be attributed to inexperience in the 

 case of the earlier one, the greater care bestowed on the latter, — and 

 above all to the advantage of local situation, and the very great supe- 

 riority in respect both of light and of defining power in the telescope 

 at the latter period. There is only one particular on which he is at 

 all inclined to insist as evidence of change — and this refers to the 

 situation and form of one of the branches of the nebula. 



The other nebula to which we have adverted — that about the star 

 1} Argus — is a far more extensive object. The space which it occu- 

 pies in the figure extends in right ascension from— 3™ 45^ to 4- 3'" 0* 



