4 



Prncpeding.s of the Uoi/al Iri^Ii Acadoin/. 



many small stars. Probably V. 17 is part of tlie dill'uscil iieb. of 

 M. 33.'" Eut wlien Lord llosse's six-foot reflector was turned to this 

 object, it was at once found to belong to the interesting class of spiral 

 nebulae, the first discovery of which is one of the greatest triumphs of 

 that instrument. In the late Lord Rosse's paper in the Phil. Trans, 

 for 1850, there is a sketch of the central portion by Mr. Johnstone 

 Stoney(pl. xxxvi., fig. 5), which does not profess to be very accurate, 

 but only to represent sufficiently well the general character of the 

 central portion. A drawing of the whole nebula, made by a subsequent 

 observer at Birr Castle, Mr. E. J. Mitchell, in 1857, appeared in the 

 Phil. Trans., 1861, plate xxvi. It gives a very good general idea of 

 the nebula ; but the photographs now available show vastly more 

 detail. The round nebula shown on the northern arm (at the very 

 top of the plate) is III. 150, while the detached ones preceding (to 

 the right) are N. G. C. 595 and 592, independently found by d' Arrest; 

 and the " knot," almost due north of the centre, is Pigourdan's 

 jN'o. 131 (my Index Catalogue l^o. 142). 



These are the only two drawings hitherto published — a fact not to 

 be wondered at, considering tlie extreme difficulty of seeing the object 

 well. But photography has now given us the means of depicting 

 faithfully the most complicated objects in the world of nebulie ; and 

 nowhere is its superiority over tlie old method of drawing more 

 clearly shown than in the case of tlie nebula now under consideration. 

 The absence of a well-defined nucleus has also prevented this nebula 

 from being successfully observed by the various astronomers who 

 during the last fifty years have made micrometric observations of 

 nebulae, though several of them (including myself) have attempted it. 



Micrometer measures of nebulae, however carefully made, are 

 known to suffer from one great defect — the systematic errors in 

 observing transits of these objects, caused by the difference of personal 

 error as regards the nebula and the comparison star. Though this 

 difficulty may be met, either by each observer determining his own 

 personal error in the manner which I suggested in 1896,^ or by 

 abandoning altogether the use of more or less distant comparison stars, 

 bright enough to be observed on the meridian, and only measuring 

 with the micrometer screw stars appearing in the same field as the 



1 W. Ilerschel as a rule did not assign new numbers to Messier's objects. 13ut 

 this one lie calls No. 17 of his fifth class, which comprises the very large nebulas. 

 In Phil. Trans., 1818, he calls it "No. 33 of the Conuaissance des Temps." 



2 "On systematic errors in observing right ascensions of nebulae." Monthly 

 Notices, 11. A. S., vol. Ivii., p. 44. 



