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8CIOPTICON MANUAL. 1G3 bag or cylinder, subject to the same amount of pressure as the oxygen. House gas collected from the burner is mostly used, when obtainable, to save the trouble and expense of generating pure hydrogen, which is but little if any better. LIME-LIGHT ADJUSTMENTS. The lime wheel is two inches in diameter and five-eighths of an inch thick, so that the available surface at its circum- ference is six inches long by five-eighths of an inch wide. These wheels, lying one upon another, are kept dry in an air-tight box shaped to a dozen of them. Lime is less liable to crack if thoroughly dried before using, as upon a hot stove or upon live coals, or, as is usual, in the hydrogen flame. After lighting the hydrogen (vapor or gas) in moderate force, and allowing it time to heat and dry the lime, the oxygen is turned on till it nearly cuts down the flame, pro- ducing an incandescent spot on the lime wheel, which is struck obliquely on the rim just below the middle of the front, about a third of an inch from the mouth of the jet; it may be brought somewhat nearer if the mixed jet is used. The direct light cannot be seen from behind, but its comparative intensity and its distance from the jet are distinctly seen, without harm to the eyes, as reflected from the surface of the condenser. The upper part of the lime wheel and the inclined cap and door, front and back of C", Fig. 43, inter- cept the light in the direction of the open top, while the upward draft, the range of the jet, and the side screen plates (not shown in the cut) all favor the ready escape of heat. To adjust the light precisely, focus a picture upon the screen and remove it, push forward the light till a bluish ring appears upon the disk, which is made even about the centre by regulating the height by the screw S, then draw back till the blue disappears, which, if the lenses are pro-