I860.] Astronomy. 203 



II. ASTRONOMY. 



(Including the Proceedings of the Royal Astronomical Society.) 



It has long been a matter of some doubt whether any perceptible disc 

 could be observed where stars of the first magnitude were examined 

 under very high powers in large telescopes. In most works attention 

 is given to the apparent diameters of these stars, and the importance 

 attached to the measure of this amount is daily felt in practical 

 astronomy, and in particular in the determination of the parallaxes of 

 stars. The most illustrious astronomers have occupied themselves 

 with this question, and Sir W. Herschel has devoted to it several 

 memoirs. According to this astronomer, the most delicate measure- 

 ments did not perhaps give an apparent diameter to Arcturus exceeding 

 the tenth of a second. This unexpected result agreed, however, with 

 the observation of J. Cassini, who had observed, in 1720, the occupa- 

 tions of y Virginis, and had concluded that the space between the two 

 stars was thirty times greater than the real diameter of each of the 

 components of this binary group.. Until the present time these two 

 celebrated observations have remained as the most characteristic types 

 of the smallness of the apparent diameters of the stars ; but since this 

 epoch the progress in the means of sounding the distant regions of 

 space has given the astronomer gigantic acromatic telescopes where- 

 with to repeat these observations. Moreover, although the method of 

 occultations is preferable to all others, yet, as our satellite does not 

 cover by its movement in the celestial sphere the most brilliant of the 

 stars, further and more recent observations were desirable, in which 

 new methods of determination should be employed. M. Chacornac 

 has recently followed up this inquiry, and has given us some new 

 determinations of the apparent diameter of Sirius. The process 

 adopted is as follows : — A " lunette prismatique," made by Secretan, is 

 employed, suitably provided with superposed rotary prisms, producing 

 the phenomenon of the gradual extinction of the extraordinary image of 

 a star ; the two images of Sirius are then brought into contact, so that 

 there is no interval between them. This disposition effected, the second 

 image is made to decrease in intensity until it arrives at the azimuth 

 of apparition ; the interval which separates them from centre to centre 

 is then compared, taking for unity of measure the diameter of the 

 extraordinary image. Operating in this manner, M. Chacornac finds 

 that, for equality of brightness of the two images, the interval is 

 originally from centre to centre equal to any one of the diameters of 

 the image, whilst it is from five to six times the diameter of the 

 extraordinary image when the images are most dissimilar in brightness. 

 If the power be then doubled, and the operation is performed in the 

 same way as for the lower power, without making any change in the 

 prismatic arrangement, the ratio one-fifth becomes suddenly one- 

 twelfth. The result of numerous measurements made with telescopes 

 of very different power is, that it has always been possible to reduce 

 the extraordinary image of Sirius, even in using the highest powers, to 

 an imperceptible point. The apparent diameter of Sirius, as that of 



