In order to make comparative tests, the same apparatus may be supplied with continuous instead of alternating currents. The after current in the former case, however, is much smaller, consisting of one electrical impulse only at each break of the commutator, whereas in the alternating system these impulses are practically continued; the result being that, all things being equal, a far greater number of lamps may be used in the shunt than when supplied by continuous current only, and it would appear that this difference can only be attributed to the fact that the rotatory motion of magnetic molecules, or polarity of the magnet, E, acquires momentum when acted upon by a suitable physical cause, such as alternating currents of electricity; this momentum lasting a sensible time after the cessation of the acting cause. If we had the gift of magnetic sight, and could see what is going on in the electro-magnet when it is excited by alternating currents, we should probably see the molecules or polarities tumbling over each other at an enormous rate. I do not think, however, that we should see anything but a vibratory motion as regards the iron molecules.--_Elec. Review_. * * * * * [AMER. MICROSCOP. JOUR.] LIGHTON'S IMMERSION ILLUMINATOR The following extremely simple plan for an immersion illuminator was first brought to the notice of microscopists a few years ago, and, in the absence of the inventor, was kindly described by Prof. Albert McCalla, at the meeting of the American Society of Microscopists, at Columbus, O. It consists of a small disk of silvered plate glass, c, about one-eighth of an inch thick, which is cemented by glycerine or some homogeneous immersion medium to the under surface of the glass-slide, s. Let r represent the silvered surface of the glass disk, b, the immersion objective, f, the thin glass cover. It will be easily seen that the ray of light, h, from the mirror or condenser above the stage will enter the slide and thence be refracted to the silvered surface of the illuminator, r, whence it is reflected at a corresponding angle to the object in the focus of the objective. A shield to prevent unnecessary light from entering the objective can be made of any material at hand, by taking a strip one inch long and three-fourths of an inch wide and turning up one end. A hole not more than three-sixteenths of an inch in diameter should be made at the angle. The shield should be placed on the upper surface of the slide, so that the hole will cover the point where the light from the mirror enters the glass. With this illuminator Möller's balsam test-plate is resolved with ease, with suitable objectives. Diatoms mounted dry are shown in a manner far surpassing that by the usual arrangement of mirror, particularly with large angle dry objectives. Ottumwa, Ia. WM. LIGHTON. [Illustration: LIGHTON'S ILLUMINATOR.] * * * * * FOUCAULT'S PENDULUM EXPERIMENTS.