716 



As was explained above, cfvslals showing various orientations of 

 the principal axis in tlie Inirizontai plaiie contribute to the formation 

 of the infralateral arc. 



Tiiat is why I calcnhited what position in space the axis ought 

 to present in order to give rise to tlie piienoinenon as it was observed. 

 1 supposed, tliat the refraction took place in the normal plane — 

 for in this case the deviation is a minimum and the intensity of 

 light a maximum. 



We consider the spherical triangle ZSN, formed by the zenith 

 Z, the sun Ö and the vanishing point of the crystal-axis N. We know 

 ZÖ, the complement of the suns-height, ^S, the supplement of the 

 angle A we already determined, and arc SN. The latter is the angle 

 of incidence / of the rays of light and is to be deduced from the 

 observed A. Arc ZN and ^Z may then be calculated, ZN gives 

 the height of the vanishing point, ^Z is the difference in azimuth 

 with the sun. From this follows the azimuth of the axis, as the sun's 

 azimuth is known. 



The results are as follows: 



Hence in the mean tlie crystal-axis is inclined at an angle of 3°. 3 

 to the horizon and its azimuth is N 71.8 W. 



The position of the axis appears to be stationary. The differences 

 with the mean value are as a rule below 1". The conclusion is the 

 more remarkable for the azimuth, as the difierence in azimuth witli 

 the sun decreases more than 7° during the observations. 



In trying to find an explanation of such a position by taking 

 into account the influence of gravitation, wind ') and atmospheric 



') M. Pinkhof. Bijdrage tot de theorie der halo-verschijnselen. Verhandelingen 

 Kon. Akademie van Wetenschappen Ie Sectie, Dl. 13, N". 1, p. 21, 1919. 



