[Illustration: Fig. 295. A. The electric light. B B. The two sets of lenses for the two pictures. C. The dissolving mechanism. D. The picture on screen.] When dissolving views are required on a grand scale, the lenses must be exceedingly large, and the condenser (corresponding with the "bull's-eye" of the simple magic lantern) should be at least nine or eleven inches in diameter, and the front glasses must be of a superior make. The lenses for a large lantern lit by the oxy-hydrogen light, are arranged as in the next cut. (Fig. 296.) [Illustration: Fig. 296. A. The lime light. B. The condensers. C. The picture. D D. The front lenses for focussing, with rack-work.] At the Polytechnic the author had no less than six lanterns working at or about the same time, to produce effects, in the views illustrating the voyages of Sinbad the Sailor; and in order to obtain the increased [Page 306] results required for dioramic effects, such for instance as the Siege of Delhi, showing the bursting of the shells, &c., the four fixed lanterns (the fronts of which are shown in the next cut) were always employed. The two upper lanterns are dissolved by discs of brass worked by the hand, and the lower ones with the fans. (Fig. 297.) [Illustration: Fig. 297. Fronts of the four lanterns, showing how the dissolving mechanism is arranged.] "Behind the scenes" always has a great attraction for young people; we have, therefore, in the frontispiece, with the help of Mr. Hine (who painted a great number of the photographs shown at the Polytechnic during the author's management), given a section of the large theatre taken whilst the effective scene of the Siege of Delhi was in progress. The optical effects were assisted by various sounds in imitation of war's alarms, for the production of which, more _volunteers_ than were required would occasionally trespass behind the screen, and produce those terrific sounds that some persons of a nervous temperament said were really stunning. In a page picture, we have also given a correct drawing of the interior of the optical box at the Polytechnic, with the four fixed lanterns, and side cupboards to hold the glass pictures. The four lanterns worked on a railway, with wheels and a circular turn-table; they could be removed, and the microscope arranged in their places. [Illustration: Before and behind the screen at the Polytechnic during the exhibition of the dioramic effects of the siege of Delhi. _p. 306_] [Page 307] III. _The Oxy-Hydrogen Microscope._