ASTRONOMY. 357 



ous fine lines and much less than those whose spectra present some 

 principal rays." 



Collecting his observations, M. Montigny has formed a Catalog^ie de 

 Scintillation des pr in cipa I es liltoiks of the northern heavens. 



The most original part of M Montigny's researches concerns the in 

 fluence of the state of the atmosphere, of terrestrial magnetism, and 

 of aurorse upon scintillation. Without entering into details, we would 

 speak of the fact that the author has taken note of some of the charac- 

 teristics of the luminous curve of the scintillometer ; besides the in- 

 tensity already defined, he now observes the aspect of the circular area, 

 which appears sometimes regular (in clear weather), sometimes diltuse, 

 curved or broken (during a rain) ; he also observes the different colors 

 seen in the scintillometer, which are quite bright with a clear sky. M. 

 Montigny's article closes as follows : 



"That which exercises the most marked influence upon scintillation 

 and most modifies its characteristics is the presence in the atmosphere 

 of a greater or less quantity of water, whether disseminated in the 

 form of vapor or falling in the form of rain or snow." 



M.^Montiguy is of the opinion that the continued study of scintilla- 

 tion will furnish useful data for weather predictions. {Bull. Astran., 

 August, 1885.) 



STELLAR PARALLAX. 



Heliometer determinations of stellar parallax in the southern hemisphere, 

 by David Gill and W. L. ElMn {Mem. R. A. S., Vol. 48, Part i, 194 

 pages). — The heliometer has an aperture of 4 inches, and is fully de- 

 scribed in the Dun Echt Publications, vol. ii. The only modification 

 made by Dr. Gill to this instrument, — which he had already emi)loyed 

 in 1874 at Mauritius, at the time of the opposition of Juno, and in 1877 

 on his expedition to Ascension Island at the time of the opposition of 

 Mars, — was in the rotation of the tube in its cradle. 



The plan adopted may be explained in a few words. To refer the star 

 that is being investigated to neighboring stars situated symmetrically 

 with reference to tbe first star, near the major axis of the parallactic 

 ellipse and differing httle in magnitude; to reduce the brightness of the 

 star to be investigated suflQciently for its image to be comparable with 

 those of the comparison stars, by means of wire screens ; finally, to sub- 

 stitute differential measures for absolute ones, grouping the successive 

 observations in such a way as to obtain the distances of the principal 

 star from the two other stars at the same instant, and consequently so 

 as to obtain the relation of the difference of the distances to their sum 

 without bringing in the absolute value of the scale. 



It seems probable that systematic errors will be thus almost entirely 

 avoided, especially if several observers combine their results. Let us 

 consider the operations in detail, taking a Centauri as an example. 



There was an unusual difficulty in this case, a Centauri being a double 



