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- 



