ASTEONOMY. 375 



Photometric observations at Potsdam. — Professor Safarik lias made 

 quite an extended analysis of Dr. Miiller's Photometric Researches, wliich 

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

 These researches have been carried on since 1877, their object being the 

 study of atmospheric extinction and of the relative brightness of the 

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

 by Dr. Miiller is a Zoelluer photometer, with an aperture of 0-()o7 me- 

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

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

 correspond to the expectations based upon the diminution of the 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 that the resulting ocular 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 comparing the five following stars, which can be observed 

 at widely different zenith distances, with Polaris : a Cygni, /; Ursae 

 Majoris, d Persei, a Aurigie, 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 carves offer as a whole a 

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

 ence of climate between Potsdam and Munich, and in spite of the dif- 

 ferent processes employed in the two observations. Dr. Miiller finds the 

 mean value 0-8250 for the fraction of a star's light that 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," {BtiU, A'^tron,^ Septem- 

 ber, 1885.) 



