486 report — 1878. 



long since announced, had been more firmly established and consolidated by ac- 

 cumulated observations. The paper was illustrated by maps showing the distribu- 

 tion of isothermal lines in harmony with Professor Hennessy's law of distribution. 



11. On a new Method of maintaining the Motion of a Free Pendulum in 



vacuo. By David Gill. 



12. On Space Numhers : an Extension of Arithmetic. By B. H. Hinton. 



In the Department of Physical Science the following Papers were read : 



1. Report of the Committee for commencing Secular Experiments on the 

 Elasticity of Wires. See Reports, p. 103. 



2. A New Form of Polariscope. 

 By Professor William Grylls Adams, M.A., F.B.S. 



This instrument has been constructed by Mr. S. 0. Tisley, on the principle com- 

 municated by the author to the Physical Society of London (see ' Proc. Phys. Soc.' 

 Vol. i. p. 152, and 'Phil. Mag.,' July 1875). The advantages gained by it are— 

 (1) an extensive field of view ; (2) an accurate means of measuring the rings and 

 the separation of the optic axes in biaxal crystals. 



The polarizer is a Nicol's prism, capable of giving a clear parallel beam of 

 polarized light 2\ inches in diameter ; this beam falling on a system of lenses, is 

 brought to a focus at the point where the crystal is placed. The beam, after passing 

 through the crystal, is rendered parallel again by another system of lenses, and 

 passes through a similar Nicol's prism and another lens for focusing upon the 

 screen. The peculiarity of the instrument consists in the arrangement of the two 

 central lenses, one on either side of the crystal. These two lenses are plano- 

 convex — very nearly hemispheres — and, with their flat surfaces inwards, form the 

 two sides of a box to hold oil or a liquid ; they are so placed that their convex 

 surfaces form portions of the same spherical surface. The crystal is placed in the 

 box at the centre of curvature of the spherical surfaces of the two lenses. 



For measurement, the crystal is immersed in oil, and adjusted to its right posi- 

 tion by a cup and socket motion ; the box and the crystal with it is then turned 

 about an axis at right angles to the direction of the axis of the beam of light, and 

 thus, either of the optic axes of any crystal may be made to coincide with the 

 centre of the field of view, the angle through which the box is turned being 

 measured to minutes by means of a circle attached to it, and a vernier attached to 

 the fixed stand supporting the instrument. A table-polariscope on the same prin- 

 ciple has also been made by Herr Schneider, of Vienna, from the description given 

 in the ' Philosophical Magazine.' By means of these instruments, both optic axes of 

 topaz are brought well into the field of view at the same time ; and by turning the 

 circle which carries the box any two directions in the crystal within an angle of 96° 

 of one another can be brought into the centre of the field of view, and the angle 

 between them accurately measured. The field of view is only limited by the in- 

 ternal reflection at the plane surface of the lenses next to the rotating lenses. 



