ASTRONOMY. 375 



Photometric observations at Potsdam. — Professor Safarik has made 

 quite au extended analysis of Dr. MUller's Photometric Researches, which 

 have appeared iu the Publications of the Potsdam Observatory (1883). 

 These researches have been carried on since 1877, their object beinjifthe 

 study of atmospheric extinction and of the relative brightness of the 

 phinets and of a series of variable stars. The instrument employed 

 by Dr. Miiller is a Zoellner photometer, with an aperture of 0*037 me- 

 ter. The aperture of the objective may be reduced by diaphrajims; 

 it seems, however, that the efiect of the diaphragms does not always 

 correspond to the expectations based upon the diminution of tlie aper- 

 ture. Professor Safarik thinks the cause of this disagreement must be 

 sought in the employment of a magnifying power that is much too low, 

 so tliat the resulting ocu»lar ring is larger than the aperture of the pupil 

 of the eye. This photometer is not suitable for colorimetric researches. 



Dr. Miiller's observations relative to atmospheric extinction have 

 consisted in couiparing the five following stars, which can be observed 

 at widely different zenith distances, with Polaris : a Oygni, 77 Ursae 

 Majoris, d Persei, a: Aurigje, and a Tauri. Curves have been traced rep- 

 resenting the logarithmic intensities as a function of the zenith distance. 

 These curves show no indication of variability for the six stars in ques- 

 tion, unless it be in the case of a Tauri. It is a curious fact that the 

 white stars do not become quite as much fainter in the neighborhood of 

 the horizon as the red stars do. This is probably due to some physiolog- 

 ical cause. We give here some of the values finally adopted for reduc- 

 tion to the zenith (the numbers given are the logarithms) : 



Four units in the first decimal place being equivalent to one magni- 

 tude, we see that for Z. D.=:80o, 86°, 88°, the reduction represents 1, 2, 

 and 3 magnitudes respectively. Dr. Miiller's curves offer as a whole a 

 very satisfactory agreement with those of Seidol, in spite of the differ- 

 ence of climate between Potsdam and Munich, and iu si)lte of the dif- 

 ferent processes employed in the two observations. Dr. M iiller linds the 

 mean value 0-8250 for the fraction of a star's light tliat comes through 

 our atmosphere when the star is seen in the zenith ; the atmosphere 

 then absorbs a little more than ^ of the light. The coefficient of extinc- 

 tion should thus be 0-8250 (0-21 magnitude); Seidel found it to be equal 

 to 0-100 (0-25 magnitude). Professor Safarik speaks in conclusion of 

 the recent work done by Wolf, and Langley." {Biill. Astroii.^ Septem- 

 ber, 1885.) 



