22 Proceedings of the 



caused by the small globules of water in the air. As the diminu- 

 tion in size of the coronae indicates an increase in size of the 

 watery spheres which cause them, this may be regarded as a 

 token of approaching rain, which falls when the particles are no 

 longer able on account of their size to float in the air. Halos 

 are a less certain indication of a storm, though if their bright- 

 ness is considerably obscured, they are not unfrequently followed 

 by rain or snow. 



The foregoing explanation of the cause of halos receives 

 confirmation from the polariscope, which shows the light to be 

 partially polarized in a plane tangent to the circle. 



SYNOPSIS. 



Positions of the Prismatic Crystals. 



I. Prisms with axes at all angles. 

 II. Prisms with axes vertical. 

 III. Prisms with axes horizontal. 



I. 



1. Primary halo by angles of 60°. 



2. Secondary halo by angles of 90°. 



II. 



1. Lateral parhelia (both primary and secondary) by 

 angles of 60°. 



2. Circumzenithal arc by angles of 90°. 



III. 



1. Tangent arcs and parhelia at upper and lower points 

 of first halo, by angles of 60°. 



2. Tangent arcs on the right and left of the lower half 

 of second halo, by angles of 90°. 



II., III. 



Horizontal white band by reflection from vertical surfaces. 



Vertical white band by reflection from horizonal surfaces. 



The paper was illustrated by black board drawings and pre- 

 parations on glass. It was followed by a discussion engaged in 

 by E. C. Seaman, Dr. Sager and Prof. Ten Brook. 



