57 
‘< For the position of the moon I am indebted to our Pre- 
-sident, Dr. Robinson. 
«‘ The place of observation may be taken as— 
Latitude, 53° 22’ 29” 
Longitude, 0° 25™ 12s 
Then at 12, night, of 10th-11th February, 1854,— 
Zenith distance of moon’s centre = 42° 38’ 29” 
Parallax, + 1 Gua 
Refraction, = an 
Apparent zenith distance of 
moon’s centre,. . . . . . 438° 14’ 33” 
Semi-diameter, + 14’ 45” 
This would give the apparent internal diameter of the ring 
about 863 degrees, which would bring that of the densest 
and most highly illuminated part, at or near the yellow light, 
to about 90 degrees. 
‘¢ This halo may, therefore (as suggested by Dr. Robinson), 
have been one of those formed by two refractions. The ab- 
sence of any inner ring, however, throws some doubt upon 
this. 
‘‘The determination of the precise angles subtended by 
these halos is of interest. Dr. Young having shown that 
they depend upon refraction and reflexion from minute crys- 
tals of suspended ice, and Arago’s experiment having proved 
that the light which passes has been polarized by refraction, 
it follows that the angle of the primary form of ice being as- 
sumed = 60° or 30°—measurements of this character afford 
the means of obtaining the possible secondary crystalline forms 
of ice crystals suspended in the higher regions of the atmos- 
phere, and there produced under conditions likely to extend 
our knowledge, as yet so very limited, of some of the forces 
upon which secondary crystalline forms depend for their pro- 
duction. : 
