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164 SCIOPTICON MANUAL. perly matched and in line, will be just before the margin would become tinged with yellow. When ready to project the views, intensify the light, if necessary, by letting on more hydrogen followed by more oxygen, till exactly the best effect is produced. During the exhibition, when the lime begins to fail at the point exposed, lifting up the back of the wheel about half an inch will bring down a fresh but hot surface into place, which may be done to advantage once in five or ten minutes, more or less, according to the quality of the lime and the force of the jet. The wheel lies loosely in its holder, and being a non-conductor, is not very hot at the back, so it can be con- veniently turned by the thumb and finger. RATIO OF SPACES. In the ordinary Sciopticon arrangement, the spaces in inches are about 3 from the focus of illumination to the first face of the condenser, 2 through the condenser, 5 be- tween the condenser and objective, and 3 through the ob- jective tube. The focus of the condenser is 3, or the back focus, 2; that of the objective is 51, or from the back glass, 41-. The conjugate foci of the condenser, measuring from the light to about 3 inches front of the objective, are 4 and 12. The conjugate foci of the objective, measuring from the picture slide to the screen, 20 feet away, are about 6 and 240. Multiplying the diameter of the slide picture 3 by the ratio ^, gives 120, or 10 feet; so, in general, the diameter of the disk on the screen is about half the measure of its distance. For example: if the screen is 9 feet wide, the instrument may be 18 feet away. If the slide pictures vary in size, the images, of course, will vary in proportion. With the long focus arrangement, for long distances, a condenser is used of 2* inches back focus, and an objective of 10 inches back focus. With these lenses the diameter