38 
the evidences of tangential polarization are frequently more or 
less masked, especially near the ends of a horizontal diameter, 
by the atmospheric polarization the direction of which is radial. 
The polarization of a halo of 22° observed by myself at Rosen- 
laui, in August last, was very distinct, and did not appear 
to be sensibly enfeebled at any part of the circle by the polar- 
ization of the atmosphere. This was probably due to the dimi- 
nished amount of atmosphere above the place of observation, 
the height of which was about 1340 métres above the level of 
the sea. The polariscope employed was Savart’s. On account 
of the small breadth of the ring of light it was difficult to deter- 
mine the plane of polarization by noting whether a dark or 
a light band occupied the middle place in the system of bands. 
But by holding a piece of varnished paper at a proper in- 
clination, so that when viewed through the polariscope it ap- 
peared to be in contact with the arc of the halo, the dark 
bands parallel to the radius of the halo produced by the light 
reflected from the paper, coincided with the hight bands seen 
in the halo. Consequently, the light from the paper being 
polarized by reflexion in the direction of a radius of the halo, 
the light of the latter was polarized in the direction of a 
tangent to the arc, i.e. was polarized by refraction. 
PROFESSOR CHALLIS stated that the explanations of the phe- 
nomena of halos were simple, and ought to be included im the 
University course of Mathematical study ; he also adverted to the 
importance of the instance of the halo formed on the ground. 
Mr Potter (Queens’) gave a brief sketch of the history of 
the observation of prisms of ice. He mentioned that in the 
course of a voyage from New York a halo 22° was observed 
in a cloudless sky, which was followed by stormy weather. 
Further Experiments on the Synthesis of Organic Acids. 
By A. BR. Catron, M.A., F.RS.E., St John’s College. 
