THE

LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FIFTH SERIES.]

MARCH 1878.

XXIV. On Reflection of Polarized Light from the Equatorial Surface of a Magnet. By JOHN KERR, LL.D., Mathematical Lecturer of the Free-Church Training College, Glasgow. IN trying to carry forward the magneto-optic inquiry which

formed the subject of my last communication to this Magazine †, I proceeded to examine a lateral face of an intensely magnetized iron bar as a reflector, and had the pleasure of obtaining good effects in the first trial. I have lately performed a series of careful experiments on the subject; and propose to give an account of these and of their very interesting results in the present paper. I mean to describe the experiments at sufficient length, for the guidance of any one who would like to repeat them. Most of them are, I think, rather easier and more satisfactory than those described in my former paper.

1. Apparatus.-The electromagnet is the same upright horseshoe that was used in my former experiments, a small but effective instrument constructed by Mr. Ladd. Each coil contains about 200 turns of thick double wire. The coils are

put into circuit, through a good Ruhmkorff's commutator, with a series of six Grove's elements, the connexions being made in the usual way as for magnetization of the horseshoe.

The reflecting bar is a rectangular prism of soft iron, 7 inches long, 2 wide, thick. The iron was selected and specially forged; and its structure is homogeneous and very

* Communicated by the Author.

"On Rotation of the Plane of Polarization by Reflection from the Pole of a Magnet," May, 1877.

Phil. Mag. S. 5. Vol. 5. No. 30. March 1878.

M

fine. One lateral face of the bar (7 by g) was planed and carefully polished by a skilled workman.

It may be worth mentioning that three such bars were forged at the same time, were cemented together, planed and polished in block, and then separated. The middle one was kept as the best reflector; but most of my results were verified upon each of the three.

2. Arrangements.-The electromagnet stands upright upon a solid table; the reflecting bar lies flat and stably on the poles of the horseshoe, in the position of an armature, its length horizontal, and its polished face vertical; the two Nicols and the lamp stand upon the same table as the magnet, and at the same height as the mirror. The diagram shows all the pieces, in horizontal section through the lamp L and the

observer's eye E. N is the first Nicol, C the point of incidence on the reflector AB, and N' the second Nicol. The poles of the horse-shoe, below the bar, are indicated by the dotted circles. The piece P between the lamp and the first Nicol was often found useful in the more delicate observations: it is a metallic screen, containing a long horizontal slit about of an inch wide. Sometimes the flat flame L has its edge presented to C; and then, the piece P being in position, the object seen at C through N' is a small segment of the flame, sensibly square. But generally, except when the angle of incidence is near 90°, the width of the flame is presented; and then the object seen in the mirror is a long horizontal rectangle, uniformly illuminated, strongly outlined above and below, and bringing out small changes of small intensity very delicately.

In the diagram, the axis of the bar A B is produced through the end B next the observer; the extremity F of the axis thus produced is used afterwards as a point of reference.

In all my later observations, the values of the angle of incidence LCE were assigned beforehand as carefully as possible, but by a method which cannot pretend to great accuracy. A broad sheet of drawing-paper, which had been cut away at the proper angle through one of its corners, was laid flat on the table, and aligned against the fixed stand of