He put into this trough two platinum electrodes and a solution of sulphate of soda, but could find no effects. Eight pages of the notebook are filled with details all leading to negative results. For ten days he worked at these experiments with liquid electrolytes. The substances used were distilled water, solution of sugar, dilute sulphuric acid, solution of sulphate of soda (using platinum electrodes), and solution of sulphate of copper (using copper electrodes). The current was sent along the ray, and perpendicular to it in two directions at right angles with each other. The ray was made to rotate, by altering the position of the polariser (in this case a black-glass mirror at the proper angle), so that the plane of polarisation might be varied. The current was used as a continuous current, as a rapidly intermitting current, and as a rapidly alternating induction current; _but in no case was any trace of action perceived_. [Sidenote: A DIFFICULT RESEARCH.] Then he turned to solid dielectrics to see if under electric strain they would yield any optical effect. He had indeed so far back as 1838 tried the experiment of coating two opposite faces of a glass cube with metal foil plates that were then electrified by a powerful electric machine. But the experiment had no result. This experiment he now repeats with a score of elaborate variations, trying both crystalline and non-crystalline dielectrics. Rock-crystal, Iceland spar, flint glass, heavy-glass, turpentine, and air, had a beam of polarised light passed through them, and at the same time “lines of electrostatic tension” were, by means of the coatings, Leyden jars, and the electric machine, directed across these bodies, both parallel to the polarised ray and across it, both in and across the plane of polarisation; but again without any visible effect. Then he tries on the same bodies, and on water, the “tension” of a rapidly alternating induced current, but still with the same negative result. Professor Tyndall stated that from conversation with Faraday, and with his faithful assistant Anderson, he inferred that the labour expended on this preliminary and apparently fruitless research was very great. It occupies many pages of the laboratory notebook. That thirty-two years later Dr. Kerr succeeded in finding this optical effect of electrostatic strain for which Faraday vainly sought, is no reflection upon Faraday’s powers of observation. Had there been no Faraday there had doubtless been no discovery by Kerr. So far the quest had been carried on either with electric currents flowing through the transparent substance or else with mere statical electric forces, and a whole fortnight had been spent without result. Now another track is taken, and it leads straight to success. He substitutes magnetic for electric forces. [Sidenote: MAGNETO-OPTIC DISCOVERY.] “13th Sept. 1845. [Illustration: FIG. 15.]