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DIAGRAM ILLUSIONS. 17 the slot, in the same position as that in which the previous image was observed. Plateau seemed very proud of the sensation caused by his first design (drawn to his request, for by this date he was unfortunately stone-blind). It represented a devil blowing up a fire, and the effect was so striking that Plateau was led to further suggestions of a photo- graphic character, which will be referred to in their proper place. It is plain that to render this apparatus available for projection it but required a condenser behind the transparent disc and an objective in front of the opaque one, but Plateau does not appear to have suggested this, and the first application of differentially speeded discs to the purpose of projection-work appears to be \h& Lantern Wheel of Life, an instrument of con- siderably later date. The Austrian Lieutenant (subse- quently General) Franz Uchatius wrote, on the i6th February, 1851, a letter to Prokesch, the head of the Viennese optical house now known by the name of Fritsch. In this letter he refers vaguely to the glories of the Phenakistoscope having been surrendered, and his subsequent papers show that the manufacture of his instruments was entrusted to the above-mentioned firm. The first form attempted was an arrangement exactly similar in principle to Fig. 12, the light being thrown through a transparent design disc on to a screen by means of an objective, the slotted disc acting as a shutter. The loss of light proved to be enormous; figures of greater size than six inches could not be shown; and Uchatius was led to invent, and Prokesch to manufacture, an apparatus of an exceed- ingly ingenious and interesting nature, which was shown at the Vienna Academy of Sciences in 1853. In this later form the diagrams were painted on the circumference of a transparent disc, which remained stationary. In front of each design a lens was placed, C