SIGNOR N. PERINI, of Garrick Chambers, Garrick Street, has invented a planetarium, which, so far as we are aware is in all respects superior to, more vraisemblable than, any apparatus of the kind hitherto attempted. The structure, for such it really is, consists first of a hemispherical dome, fourteen feet in diameter at the base and the same in height, resting on twelve columns. Getting underneath the dome, one sees the vault overhead coloured so as to represent the starry sky, with the milky way and the constellations in their proper places. Suspended from the top by a narrow hollow rod is an opal globe lit up by gas or electricity to represent the sun, and around this, at their proper proportional distances, are suspended by almost invisible wires, the planets from Mercury to Uranus. By a slight turn of a key Signor Perini sets the solar system in motion, when the sun revolves on its axis, and all the planets in their proper elliptical orbits and at their proper axial inclination, and with proportionate velocity. Saturn has his rings and the other planets their moons; the earth, about the size of a walnut, by a mechanism peculiar to itself, revolves on its axis at a rate accurately proportioned, the same mechanism causing the moon, a small pearl, to revolve round the earth in its own proper orbit. Round the base of the dome the various signs of the zodiac are indicated, and the paths of the planets are shown by ellipses traced around the vault. The spectator is supposed to be standing somewhere under neath the solar system, and the general effect is very striking. To us it seems the most effective method hitherto devised to convey to old or young a realistic conception of the arrangement and motions of the planets. During the working of the mechanism not a sound is heard, though above the dome, and concealed from view is an elaborate arrangement of machinery. This machinery is of the nature of clockwork, with, however, a special feature by means of which the elliptical motions of the planets are effected. Inside the earth is a watchwork arrangement, which could easily be adapted to the other planets were it not for the expense. When wound the machinery can be kept going continuously for upwards of five hours; it can be stopped at any moment. The invention has, we believe, cost Signor Perini seven years' unremitting work and seven hundred pounds expenditure; We believe that the work has all been done at night and during early morning hours, as the inventor has to give his daytime to his profession of teacher. Signer Perini informs, us that he could without difficulty make his planetarium as large as the Albert Hall and small enough to become a school apparatus for teaching. He showed us a table, like a small writing-table, between the tops of which he had arranged his machinery on a small scale to give motion to a tellurium which he fits on to the table. Of course the invention, as indeed Signer Perini admits, may be capable of improvements in detail, but as it stands it seems to us a triumph of ingenuity and determined perseverance, for which its inventor deserves the highest credit.