I 



RECENT EESEAKCHES RELATIVE TO THE NEBULiE. 30i5 



I pass now to the labors of M. d'Arrest, relative to the nebulce. This 

 astrouomer had begun to occupy himself with this subject while he was still 

 attached to the obsei-vatory of Leipsic, and, since 1857, has published in the 

 collection of the memoirs of the Royal Society of Saxe the result of his first 

 observations of 230 nebuloe, made with a double annular micrometer, of Frauen- 

 hofer's construction, applied to a telescope of 52 lines opening and 6 feet focal 

 length. Prof. d'Arrest* is at present director of the observatory of Copen- 

 hagen, and he has continued, since the month of September, 1861, his observa- 

 tions of the uebulse, with a large achromatic telescope, of 11 inches opening and 

 16 feet focal length, the optic power of which he estimates to be intermediate 

 between that of Herschel's 20 feet reflecting telescope, and that of the telescope 

 of the same kind with which Lassell likewise has observed the nebula) from 

 1852 to 1854. The telescope of Copenhagen has enabled M. d'Arrest not 

 only to recognize all the nebulte of Herschel, but to discover more than a hun- 

 dred new ones among 776 observed in eight months. He has been enabled also, 

 under favorable circumstances and with some diflficulty, to see certain nebulsB 

 indicated by Lassell. 



M. d'Arrest, making his observations alone, soon perceived that he could 

 scarcely combine the observation of celestial objects of very feeble light with 

 the microscopic reading of the circles of his instrument. It follows that his 

 new catalogue will not assign, with all the precision attainable, the absolute 

 position of each object on the celestial sphere. This position is only given to 

 the minute of a degree in right ascension and in declination ; but as the nebulae 

 are very carefully compared with the neighboring small stars by the help of 

 annular «nd thread micrometers, we shall thus have competent means for ascer- 

 taining with precision their proper movements in respect to those stars, which 

 constitutes one of the principal aims of the researches of M. d'Arrest. This 

 astronomer has published, in No. lo66 of the A. N., an interesting notice, dated 

 20th May, 1862, of his later labors; and from this I shall extract some de- 

 tails, tending to complete those which precede. 



VARIABILITY OF THE BRIGHTNESS OF THE NEBULAE. 



M. d'Arrest considers as well established one of the results of the great 

 labor of Argelander, in which has originated his new catalogue of stars, namely, 

 that, of 50,000 stars already well recognized, there exists but a small number 

 of which the brightness is periodically variable ; and he believes the same 

 may be aifirnied, though with less certainty, to be very nearly the case Avith 

 the nebulae. 



Sir W. llerschel had subdivided the nebulfe into three classes, with reference 

 to their relative degree of brightness. M. d'Arrest has found a great many 

 instances in which the ncbulaj, as at first classed by Herschel, must now be as- 

 signed by one or even two units a new place in the classification. Herschel 

 himself had, in the course of some years, changed several of his appreciations. 

 But in view of the great diversity of atmospheric influences in humid climates, 

 bearing upon observations of this kind, M. d'Arrest thinks, like M. Otto Struve, 

 ihat it is impossible to be too circumspect in regard to the conclusions to be de- 

 duced from variabilities of this nature. He instances, however, a small num- 

 ber of cases in which some degree of variability has been positively ascer- 

 tained. 



The first of these cases is that resulting from the observations of M. 0. Struve 

 on the nebula of Orion before spoken of. The observations of this nebula 

 recently made, at difi"erent times and in favorable nights, by M. d'Arrest, with 

 his large telescope, have confirmed those of M. Struve, particularly as regards 



* See M. N., vol. xvii, p. 48. 



