RECENT EESEARCHES EELATIVE TO THE NEBUL.E. 301 



M. O. Struve proceeds to mention in detail four parts of tlic nebula of Orion 

 in which he perceived most distinctly, in an interval of some months, changes 

 of form or of the degree of light. The first is a bay, extending from the straits 

 of Lc Gentil in the direction of the .trapezium of stars situated towards the 

 middle of the nebula. 'J^his bay appeared to him at one time altogether 

 obscure, like the straits ; at another, full of nebulosity, and little inferior in 

 brightness to the surrounding portions of the region of iluygens. Dr. Lamont 

 first delineated this bay, which has never been seen by Sir John llerschel. 

 The second is a nehulous bridge, which crosses the great straits, with a point 

 of concentrated light about midway. ]\[. Struve saw it in winter, sometimes as 

 represented by llerschel, sometimes as by LiapounofF, with much greater con- 

 centration of light, but always much more extended than in the representations 

 of these astronomers, and closely approaching the southern limit of the great 

 strait. Very faint traces of it arc indicated by M. Lamont, while Professor 

 Bond did not see it at all. The third is a nebulosity surrounding star 75 of 

 Herschel's catalogue, which appeared to M. Struve to be subject to great 

 changes of brightness. Lastly, the fourth part is a sort of narrow canal, uniting 

 in a right line the obscure space situated around the stars 76, 80, and 84, of 

 Herschel's catalogue, with the north side of the great strait, near the exterior 

 extremity of the bridge before mentioned. The canal, which has not been 

 represented by any other observer, was distinctly seen by M. Struve March 24, 

 1857, Avhile on other occasions he has not perceived the least trace of it. 



This astronomer, in closing his communication, adds, that the general im- 

 pression resulting from his observations is to the effect that the central part of 

 the nebula of Orion is in a state of continual change of brightness as regards 

 many of its portions. In those cases where the images were most distinct, 

 their appearance did not seem entirely uniform from night to night. Those 

 changes in the degree of light cannot, however, be perceived in the greater 

 number of cases without instruments of considerable optical power ; and he does 

 not think that achromatic telescopes of less than ten inches opening can serve 

 to verify them, except under atmospheric conditions extraordinarily favorable. 



The twenty-second volume of the M. N. (pp. 203-207) contains the analysis 

 of another memoir relating to the same nebula. It was communicated to the 

 Astronomical Society, May 10, 1861, by Professor George Bond, who has suc- 

 ceeded his father in the direction of the observatory of Harvard College, at 

 Cambridge, near Boston. The paper bears for its title. On the spiral stritcture 

 of the great iiehula of Orion. 



Professor Bond the father, in a memoir published in 1848, had already 

 remarked that the light of this nebula seemed to present a radiated appearance 

 on its southern side, starting from the neighborhood of the trapezium of stars 

 situated towards its middle. Professor G. Bond has undertaken, since 1857, 

 to form a catalogue of the stars comprised in a square of forty minutes to the 

 side, having of Orion lor its centre. He selected one hundred and twenty-one 

 bright stars as guiding points to which to refer the smaller stars, of too feeble 

 light, for the most part, to remain visible under a strong illumination of the 

 micrometric threads. In a first sheet he has arranged two hundred and sixty- 

 two stars, and then subdivided the same surface into four charts, finally reunited 

 into a single one. The form and arrangement of the elongated luminous tufts, 

 alternating with the more obscure spaces stretching from the neighborhood of 

 the trapezium, have been determined by two independent procedures, the nebula 

 being first delineated as a bright object on a dark ground, and then as a dark 

 object on a white ground. 



I cannot enter here into the descriptive details given in the analysis of Prof. 

 Bond's memoir, and I shall confine myself to a report of its conclusion. The 

 general aspect of the greater part of the nebulae of Orion is an assemblage of 

 tufts or curvilinear pencils of luminous matter, emanating from bright masses 



