144 Intelligence and Miscellaneous Articles. 



from each other ; the difference of appearance arises from a variation 

 in the direction of the corona in respect of the observer. The 

 aurora in the form of curtains or of draperies are very long and 

 high, but have no appreciable thickness. They may sink to within 

 600 metres from the ground. Zones and coronse, on the contrary, 

 are always at very great heights and may exceed 320 kilometres. 

 Thus, in the south of Greenland, the region in w T hich the aurora are 

 produced extends from the highest regions of the atmosphere to 

 the surface of the soil, w T hile in temperate countries the phenomenon 

 takes place simply in the higher regions of the air. If, then, we 

 admit the electrical origin of the phenomenon, this electric current 

 circulates in low latitudes in the higher regions of the atmosphere 

 and produces the well-known appearance of rarefied gases, while 

 in the zone proper of aurora boreales it descends to the ground. The 

 vertical direction of this current in arctic countries, together with 

 the variations of density of the air traversed, must be the cause of 

 the great difference of this phenomenon in arctic and in temperate 

 regions. 



The usual colour of the aurora borealis is white with a faint 

 tinge of green or of yellow. The cloud forms have a greyer colour. 

 The edges of the draperies are often momentarily coloured red or 

 green. 



At Godthaab the greatest daily activity of the aurora is at 9 p.m., 

 and the annual maximum is about the winter solstice. The series 

 of the observations in Greenlaud show a maxim am frequency at 

 the periods when the number of sun-spots is smallest, and conversely. 



The law enunciated by M. Trombolt, according to which the 

 auroral zone moves in the course of twenty-four hours so that 

 during the night it is directed to the north, seems contradicted by 

 experience. 



Lastly, a well-defined distinction has been observed between the 

 greatest frequency of aurora boreales in arctic and in temperate 

 countries. If this is a general fact, it will prove that a more active 

 evolution of the aurora borealis in low latitudes enfeebles the 

 auroral activity in the characteristic region of the aurora. — Journal 

 cle Physique, February 1892 ; from Observations Internationale 

 polaire ; expedition danoise: Copenhagen 1891. 



ON THE ABSORPTION OF LIGHT IN TURBID MEDIA. 

 BY A. LAMPA. 

 In front of the collimator-slit of a Glan's photometer a glass 

 trough with parallel sides was placed, and the lower half filled with 

 an emulsion of a definite composition of an alcoholic solution of 

 mastic (4-062 to 5*592 gr. in 100 gr. absolute alcohol) in water. 

 The absorption was examined which the parallel rays experienced on 

 passing through the emulsion, and in twenty places in the spectrum. 

 The observations agree very well with Lord Rayleigh's formula. 

 The author estimates the diameter of the emulsion at less, than 

 0-2 /x. On comparing emulsions in which the product of thickness 

 and density was constant, the weaker media seemed to absorb more 

 strongly. — Beibldtter der PhysiJc, No. 4, 1892; from Wiener Berichte. 



