“The heavy glass had tinfoil coatings on its two sides but when these were taken off the effect remained exactly the same. “A mass of soft iron on the outside of the _heavy glass_ greatly _diminished_ the effect [see Fig. 17].... “All this shews that it is when the _polarized ray_ passes _parallel_ to the _lines of magnetic induction_ or rather to the _direction of the magnetic curves_, that the glass manifests its power of affecting the ray. So that the heavy glass in its magnetized state corresponds to the cube of rock crystal: the direction of the magnetic curves in the piece of glass corresponding to the direction of the optic axis in the crystal (see Exp. Researches 1689–1698).... [Illustration: FIG. 18.] “Employed our large _ring electro-magnet_ which is very powerful and has of course the poles in the right [position] only they are very close not more than [0·5] of an inch apart. When the _heavy glass_ was put up against it the effect was produced better than in any former case.... [Sidenote: ENOUGH FOR TO-DAY.] “Have got enough for to-day.” The description which he published in the “Researches” of the first successful experiment is as follows:-- “A piece of this glass about 2 inches square and 0·5 of an inch thick, having flat and polished edges, was placed as a _diamagnetic_[47] between the poles (not as yet magnetized by the electric current), so that the polarized ray should pass through its length; the glass acted as air, water, or any other indifferent substance would do; and if the eye-piece [_i.e._ analyzer] were previously turned into such a position that the polarized ray was extinguished, or rather the image produced by it rendered invisible, then the introduction of this glass made no alteration in that respect. In this state of circumstances the force of the electromagnet was developed, by sending an electric current through its coils, and immediately the image of the lamp-flame became visible, and continued so as long as the arrangement continued magnetic. On stopping the electric current, and so causing the magnetic force to cease, the light instantly disappeared; these phænomena could be renewed at pleasure, at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect.” He paused for four days in order to procure more powerful electromagnets, for the effect which he had observed was exceedingly slight: “A person looking for the phænomenon for the first time would not be able to see it with a weak magnet.” The entry in the notebook begins again:-- “18th Sept. 1845. “Have now borrowed and received the Woolwich magnet.” [Sidenote: AN EXCELLENT DAY’S WORK.]