713 



during their railway journey between Tjiinalii and Bandoeng. However 

 they saw the small arc not on the left (west) of the sun, but on 

 the right (east). Though on the left liand side the sky presented an 

 equally smooth Cirrus-veil as on the right, nothing was to be seen 

 there. According to Rijken Rapp the arc was intensely coloured and 

 benl like a portion of the greater ring. 1 have not been able to note 

 any curvature at Lembang. 



Before discussing my measurements I give here a short review 

 of the theory of the infralateral arc. 



Bkavais explains the arc by the refraction of light in ice-crystals 

 with a horizontal principal axis, the light entering at a vertical basal 

 plane (the hexagonal terminal plane of the crystal) and leaving at 

 a sideplane of the prism. The refracting angle is 90° then. For a 

 definite position of the principal axis (defined f. i. by its azimuth) 

 we get a circular arc perpendicular to this axis and at a distance 

 from the sun, depending on the sun's height. In a simple way we 

 may imagine this circle by drawing the case of the circumzenithal 

 an; and rotating the drawing then over 90°, so that the axis which 

 at first was vertical, now gets a horizontal position. To each azimuth 

 of the axis such a circle belongs. The envelope of all these minor 

 circles is the infralateral arc. One among these circles is tangent to 

 the greater ring. For the rest, this arc does no more then the 

 circumzenilhal arc fulfil the conditions for minimum deviation of 

 the refracted rays of light. 



Pernter (Meteorologische Optik, 1'* Edition) sticks to these conditions. 

 He considers the arc as a "Lowitz arc of the greater liiig" and 

 deduces the form and position of the lateral arcs to the smaller and 

 greater rings in an exactly analogous way. Without going into the 

 details of the calculations, we may state, that the arc according to 

 Pernter in consequence of the conditions for minimum deviation 

 which he imposes, generally will be less distant from the ring 

 than Bravais's arc. 



Besson (Sur la Theorie des Halos, Paris 1909, p. 51, p. 70) has 

 shown, that Pernter's theory is not very satisfactory. Exner (Pernter- 

 ExNER, Meteor. Optik, 2"'' Edition 1922 p. 405) concurs in this 

 opinion and develops a new theory. During the normal fall of an 

 ice-prism the principal axis and one of the bigger diagonals of the 

 hexagon are placed horizontally. An infralateral arc may than be 

 formed by light, entering the basal plane and emerging from one 

 of the oblique prism-planes. The plane perpendicular to the refracting 

 edge is inclined to the horizon at an angle of 30°. For one definite 



47 



Proceedings Royal Acad. Amsterdam. Vol. XXVI. * 



