20 



which was sharply defined and distinct throughout, except at 

 the ends of the arms, where its light gradually melted away. 

 On looking through the numerous notices of paraselenae, which 

 are to be found in the early volumes of the Philosophical 

 Transactions, I find but one, seen by Hevelius at Dantzic, 

 in the year 1660, which was similar to the present phenome- 

 non, or to that of the preceding month described by Dr. Ro- 

 binson. The non-appearance, in any instance, of a complete 

 vertical circle, seems to forbid the supposition that this cross 

 can have been produced, like the horizontal white circle, by 

 reflection from the facets of the prisms of ice. It is probably 

 a phenomenon of diffraction ; and indeed it is described by one 

 of the gentlemen who witnessed it, as resembling the cross of 

 light which one sees, in looking at the sun or any bright ob- 

 ject, through the silk of an umbrella. It would be impor- 

 tant, with reference to the physical explanation of the pheno- 

 mena, that the light, both of the horizontal circle and of the 

 cross, should be analyzed with a tourmaline or double-refract- 

 ing prism. The received explanation of the former may thus 

 be easily tested ; for, it follows from the hypothesis upon which 

 that explanation rests, that the light of the circle must be 

 partially polarized in every part, the polarization increasing 

 with the distance from the moori or sun on either side, up to 

 a certain angle, at which it should be complete, and again 

 diminishing from that point to 180° of distance, where it should 

 disappear. 



A lunar halo, with a pair of false moons, similar to that above 

 described, but without the cross, was seen at Bandon, in the 

 County of Cork, on the night of the 1st of May. The appear- 

 ances are thus described by Mr. Richard Allman : " A faint 

 halo surrounded the moon, at a distance which appeared equal 

 to that of the pole star from the nearest point in the Plough. 

 In this halo, at the extremities of the horizontal diameter, ap- 

 peared two nebula-like, luminous masses, between which an 

 intermittent stream of faint light seemed to play. I first per- 



