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arrangement by which I can attach a glass plate to the side of one of the limbs of the fork, and I have placed exact counterpoises on the other limb. I now blacken the slip of glass Fig. 39.—Tuning-fork arranged for causing a smoked-glass plate, a, to vibrate, b, electro-magnet; current passes in at c, along limb of fork, d, along platinum wire to platinum contact at e, thence by wire, f, to electro-magnet, b, thence back to battery by wire from g. When b is magnetised the limbs of the fork are drawn together and contact is broken at e. The limbs then fly back and again make contact at c, and so on. (an ordinary microscopic slide) in the smoky flame of this lamp and attach it to the fork. Set the fork vibrating. I shall now write something on the smoked surface with a needle point, taking care to write at right angles to