Proceedings of the Cambridge PhihsopMcal Society. 18T 
Meeting Easter Term.^ April 17. — 1. A letter was read 
from the Rev. G. Davis to Dr Wood, detailing certain optical phe • 
nomena observed at Kilkhampton in Cornwall on April 5* 1820. 
A large halo was observed about the sun. T wo mock suns were 
at the same time seen on the halo, at the extremities of an hori- 
zontal line passing through the sun. The halo and the two 
images, which were nearly as bright as the sun, continued vi- 
sible for nearly a quarter of an hour. That on the north side 
vanished some minutes before the other. The halo appeared 
of the same breadth as the sun, and near the mock suns exhi- 
bited a series of colours in the order of the rainbow. The 
mock suns were also seen at St Gennys, about twelve miles to 
the south-west of Kilkhampton. Above the northern image, 
which was not quite so bright as the other, was an appearance 
of two coloured bows, convex to each other. At this place, 
however, no halo was visible about the true sun. 
2. A paper by J. F. Herschel, Esq. was read, On the Rotation 
impressed by Plates of Rock-crystal on the Planes of Polarization 
of the Rays of Light, as connected with certain peculiarities of its 
crystallization. Mr Herschel has found, that the two kinds of 
rotatory or circular polarisation are related to the direction oP 
the plagie drai faces in rock-crystal ; so that we can always infer 
the kind of polarisation when the crystal exhibits the smallest 
trace of the plagiedral planes. 
3. * A paper by Mr Whewell was read, On the Position of the 
Apsides of orbits of great eccentricity.” The case considered is, 
when a body revolves acted on by a central force, varying ac- 
cording to any simple law, and at its lower apside approaches 
indefinitely near to the centre. To this ultimate case, Mr W. 
applies considerations which enable him to integrate the expres- 
sion for the angle between the apsides. It appears, that when 
the law of the force is any direct power of the distance, this 
angle is a right angle. In other cases, it assumes different va- 
lues greater than this, and finally becomes infinite, when the 
force varies as the inverse cube. 
May 1. — 1. A paper by the President on Polar Navigation 
was read. 
2. A paper by Mr Herschel was read, On certain remarkable 
instances of deviation from Newton’s scale in the tints develo- 
