306 Proceedings of Philosophical Societies. [Apnit, 
At this meeting also, a paper, by Capt. E. Sabine, was read, 
entitled “ Observations on the Dip and Variation of the Mag- 
netic Needle, and on the Intensity of the Magnetic Force, made 
during the late Voyage in Search of a North-West Passage.” 
The author stated, that the dippimg needle employed in these 
observations was similar to that described by Mr. Cavendish, 
and made by the same artist. It was so adjusted, that on revers- 
ing the poles, the dip remained unaltered ; and it was’ placed in 
the direction of the magnetic meridian by a compass, placed at 
such a distance as to remain during the observation for the pur- 
pose of occasional verification. 
In determining the intensity of the magnetic force, a magnet 
was employed to draw the needle to a horizontal position. The 
magnet was then removed at an observed moment, and the 
needle permitted to oscillate till the arcs became too small to 
be observed. At every tenth vibration both the are and time 
were noted. 
The azimuth compasses ‘employed by Capt. Sabine to deter- 
mine the magnetic variation, were made upon Capt. Kater’s 
improved plan. The observations were generally made upon the 
ice, to avoid the great irregularities produced on board by the 
iron of the ship. The results of the whole of these different 
classes of observations were arranged in the form of tables. 
March 4.—A paper, by Dr. Brewster, was read, on the action 
of crystallized surfaces upon light. Malus had remarked that the 
action exerted upon light by the first surface of iceland spar is 
independent of the position of its principal section; that its 
reflecting power extends beyond the limits of the polarising 
forces of the crystal ; and that as light is only polarised by pene- 
trating the surface, the forces shih produce extraordinary 
refraction begin to act only at this limit. He also remarked, that 
the angle of mcidence at which this spar polarises light by par- 
tial reflection is 561° ; and that whatever be the angle included 
between the plane of incidence and the principal section of the 
crystal, the ray reflected by the first surface is always polarised 
in the same manner. After stating these observations of Malus, 
Dr. Brewster proceeded to observe, that his experiments upon 
the subject led him to draw different conclusions, and rather 
seemed to indicate that the polarising forces extend beyond the 
crystal. He also showed that the force of double refraction and 
polarisation originate from the surface of bodies, though its 
intensity depends upon the inclination of the surface to the axis 
of the crystal, and that the ordinary and extraordinary image 
may be extinguished at pleasure, and thus a doubly refracting 
crystal be converted into a singly refracting one. He also 
showed that the change in the angle of polarisation produced by . 
the interior force depends on the inclination of the reflecting 
surface to the axis of the crystal and upon the azimuthal angle 
which the plane of reflection forms with the principal section ; and 
that the change in the direction of the polarisation depends 
