Gautier on Nebulce. 419 



changes that attention should be directed, and they can be better 

 noted by attending to certain prominent portions of the nebula 

 than by watching it as a whole. Following this rule, M. Struve 

 thought he detected considerable alterations in a single winter, 

 and he mentioned four parts of the nebula in which they seemed 

 to have occurred. The first is a bay extending from the Strait 

 of Le Gentil in the direction of the trapezium of stars situated 

 towards the middle of the nebula. This bay sometimes 

 appeared to him dark like the strait; at others full of nebu- 

 losity, and little inferior in light to the parts surrounding the 

 region of Huyghens. Dr. Lamont was the first to describe 

 this bay, which was not seen by Sir J. Herschel. The second 

 is a nebulous bridge traversing" the Great Strait, and exhibit- 

 ing towards its centre a luminous point. In the winter M. 

 Struve saw it as represented by Herschel and by Liapounoff, 

 with much more concentration of light, but always much more 

 extended than it appeared to these two astronomers, and closely 

 approaching the southern limit of the Great Strait. M. Lamont 

 had only indicated the faintest traces of it, and M. Bond had 

 not seen it at all. The third is a nebulosity surrounding star 

 75 of HerscheFs catalogue, and which appeared to M. Struve 

 subject to great changes of light. The fourth is a sort of nar- 

 row channel {canal etroit) connecting in a straight line the dark 

 space situated about stars 76, 80, 84 of HerscheFs catalogue, 

 with the northern margin of the Great Strait. This channel, 

 not figured by any former observer, was distinctly seen by M. 

 Struve on the 24th March, 1857, but on other occasions he 

 could not discover the least sign of it. He thus arrived at the 

 conclusion that the centre of the Orion nebula is in a state of con- 

 stant change, but he considered that, except under favourable 

 circumstances, no achromatic telescope of less than ten inches 

 aperture would enable them to be perceived. 



In 1861 the Monthly Notice of the Astronomical Society 

 contained a report by Mr. G. Bond, of Harvard College, on the 

 " Spiral Structure of the Great Nebula in Orion." Mr. Bond, 

 senior, in 1848 noticed a disposition in the light of this 

 nebula to radiate from the south side, starting from the vici- 

 nity of the trapezium of stars situated towards the middle." 

 Mr. G. Bond began, in 1857, to form a catalogue of stars, com- 

 prised within a square of 40 minutes, having $ (theta) in its 

 centre. He selected 121 brilliant stars as points of reference 

 for the smaller stars which were of too feeble light to remain 

 visible when his micrometer threads were illuminated. He 

 placed in his first sketch 262 stars. The form and disposition 

 of the elongated luminous tufts, alternating with darker spaces, 

 proceeding from the neighbourhood of the trapezium, were 

 determined by two independent proceedings, the nebula being 



