“To-day worked with lines of magnetic force, passing them across different bodies transparent in different directions, and at the same time passing a polarized ray of light through them, and afterwards examining the ray by a Nichol’s Eye-piece or other means. The magnets were Electro-magnets one being our large cylinder Electro-magnet and the other a temporary iron core put into the helix on a frame. This was not nearly so strong as the former. The current of 5 cells of Grove’s battery was sent through both helices at once and the magnets were made and unmade by putting in or stopping off the electric current.” Air, flint-glass, rock-crystal, calcareous spar, were examined, but without effect. And so he worked on through the morning, trying first one specimen, then another, altering the directions of the poles of his magnets, reversing their polarity, changing the position of his optical apparatus, increasing the battery-power of his magnetising current. Then he bethinks him of that “heavy-glass”--the boro-silicate of lead--which had cost him nearly four years of precious labour during the first period of his scientific life. The entry in the notebook is characteristic. [Illustration: FIG. 16.] “A piece of heavy glass, which was 2 inches by 1·8 inches and 0·5 of an inch thick, being a silico-borate of lead, was experimented with. It gave no effects when the _same magnetic poles_ or the _contrary_ poles were on opposite sides (as respects the course of the polarised ray);--nor when the same poles were on the same side either with the constant or intermitting current; =BUT= when contrary magnetic poles were on the same side there _was an effect produced on the polarised ray_, and thus magnetic force and light were proved to have relations to each other. This fact will most likely prove exceedingly fertile, and of great value in the investigation of conditions of natural force. “The effect was of this kind. The glass, a result of one of my old experiments on optical glass, had been exceedingly well annealed so that it did not in any degree affect the polarized ray. The two magnetic poles were in a horizontal plane, and the piece of glass put up flat against them so that the polarized ray could pass through its edges and be examined by the eye at a Nicholl’s eye piece. In its natural state the glass had no effect on the polarized ray but on making contact at the battery so as to render the cores N and S magnets instantly the glass acquired a certain degree of _power of depolarizing the ray_ which it retained steadily as long as the cores were magnets but which it lost the instant the electric current was stopped. Hence it was a permanent condition and as was expected did not sensibly appear with an intermitting current. [Illustration: FIG. 17.] “The effect was not influenced by any jogging motion or any moderate pressure of the hands on the glass.