406 Proceedings of the Royal Irish Academy. 



•upon the surface at wluch. it entered the crystal, the rays then emerg- 

 ing in parallel lines. In this case ib is evident that the deviation is 

 constant, and "equal to 180° ; hence, whatever be the angle of incidence, 

 the light will return in the same direction, and thus an indefinite 

 number of crystals will contribute to the formation of an anthelion 

 opposite to the sun. This may happen whether the principal section 

 of the prisms be square or oblong, and it may also take place in hexa- 

 gonal prisms, since a section of any of these made by a plane perpendi- 

 cular to two opposite siu'faces and to the bases will have the form of 

 an oblong. But as the position of the anthelion thus formed would be 

 at or near the antisolar point, it could only occur at a low altitude ; 

 hence it would not explain the appearances which have been seen at 

 considerable heights. Mr. Barker has described one seen near Lake 

 Superior, which had an altitude greater than 45°, or above twice that 

 of the sun {Pliilosopliical Transactions for 1787, p. 44). The horizontal 

 circle which was in that instance so high is usually represented as 

 passing thi'ough the sun. 



