A. MATHEMATICS AND ASTRONOMY. 29 



to those who are engaged in drawing and studying the 

 clouds. Bulletin Hebdomadaire, XVI., 25. 



OX THE ERRORS OF MICROMETRIC MEASUREMENTS. 



In the course of an excellent series of measurements of 

 the relative positions of the stars in the cluster of Sobieski's 

 Shield, Professor Helmert has made a determination of 

 the so-called personal errors in micrometric measurement 

 of position angles. As Otto Strove and Dembowski are the 

 only astronomers who have as yet thoroughly investigated 

 this important source of error, the results of Ilelmert's ob- 

 servations are especially valuable. The apparatus used by 

 the latter can easily be applied to any telescope, and will, it 

 is hoped, be frequently applied by others. Helmert con- 

 cludes that there was possibly in his case a gradual change 

 in his method of observation, and that when this is elimi- 

 nated there still remains a periodical error in his measure- 

 ments of angles, which, although much smaller than that of 

 Otto Strove, is quite similar to it in its general features. 

 Publications of the Hamburg Observatory , No. 1, p. 19. 



ASTRONOMICAL WORK WITH THE GREAT MELBOURNE 



TELESCOPE. 



From the proceedings of the Royal Society of Victoria 

 we learn that during the past five years Mr. Ellery, of the 

 Melbourne Observatory, has examined the positions of 38,000 

 stars. The great reflecting telescope, of four feet aperture 

 and forty feet focal length, has been employed in examining 

 the star Eta Argus and its surrounding nebula. The spec- 

 trum of this star is found to be crossed by bright lines, 

 which seem to indicate that hvdroo-en, nitroo-en, sodium, and 

 magnesium are present no dark lines having been seen 

 with certainty, though they were suspected. Considerable 

 changes have, however, occurred in the spectrum, if we may 

 judge from a comparison of the records of 1869 and 1874, 

 since in the latter year no bright lines were to be detected, 

 while a distinct nebulosity surrounded the star, which had 

 formerly appeared projected on the black background. The 

 nebula surrounding this star is evidently subject to great 

 changes. In the spectrum of Jupiter absorption lines con- 

 stantly appear. An examination of the small stars near 



