356 SCIENTIFIC RECORD FOR 1885. 



uitude by Herr Palisa. Ilerr Palisa made his estiuiales with a 12 inch 

 telescope, Dr. Peters with a 13.5-iDch. Herr Palisa adds that, after 

 having used a 6 inch telescope at Pola for a long time, he found himself 

 quite unable to estimate magnitudes with his 12 inch for as much as a 

 year after beginning to use it, the differences of intensity between the 

 tenth and thirteenth magnitudes seeming much less sensible than with 

 the 6 inch. Dr. Peters's twenty charts that are already published are 

 remarkably accurate, according to the careful revision of them made by 

 Herr Palisa. {Bull. Astron., March, 1885.) 



Scintillation of stars. — The instrument employed daily by M. Mon- 

 tigny consists of a telescope of 0™-077 aperture, which contains, in front 

 of the focus, a circular piece of glass, capable of being turned more or 

 less rapidly about an axis jjarallel to the optical axis of the telescope; 

 the glass plate, when slightly inclined toward the optical axis, causes 

 the luminous rays to deviate laterally, and when it is very rapidly re- 

 volved a luminous circumference appears interrupted by colored arcs, 

 which correspond to the changes of color. The number of these arcs, 

 which is determined by means of a micrometer especially adapted for 

 the purpose, in which are two cross-threads comprising an aliquot ])art 

 of the circumference, divided by the number of revolutions made by the 

 plate in one second, gives the number of changes per second of the 

 star's color. 



Dufour seems to have been the first to make a regular series of obser- 

 vations of the changes of color in stars. Without any especial instru- 

 ment, simply comparing the stars among themselves (as is often done 

 for variable stars) Dufour came to the following conclusions : 



(1) Eed stars scintillate less than white stars. 



(2) Excepting near the horizon the scintillation is proportional to 

 the product obtained by multiplying the thickness of the air traversed 

 by the luminous ray by the astronomical refraction of the altitude under 

 consideration. 



(3) Aside from the influence of color there are essential differences in 

 the scintillation of different stars, which seem to depend upon the stars 

 themselves. 



The second law is important, as it permits of reducing all observa- 

 tions to the same zenith distance (the angle of 60° has been adopted). 



M. Montigny has confirmed the results of the Swiss investigator by 

 means of his scintillometer, and has further shown on what the scintil- 

 lation of stars may depend. According to the work of Secchi, Huggins, 

 and Miller, the stars are arranged in two great classes : White or blue 

 stars, like Sirius, whose spectra show, in particular, hydrogen bands; 

 yellow stars, like the sun, with spectra containing numerous fine lines 

 (magnesium, sodium, &c.). In grouping his observations M. Montigny 

 affirms that both for Dufour and himself: 



" The stars whose spectra are characterized by dark bands and black 

 lines scintillate less than stars whose spectra are composed of numer- 



