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- Also the auxiliarly circle is the locus of the feet of the perpendiculars from the
**foci**on any tangent. - (casale) the rustici were grouped in families (
**foci**): the tenants paid from 4 to a of the crop, besides a poll-tax and labour-dues. - If the law of attraction is that of gravitation, the orbit is a conic section - ellipse, parabola or hyperbola - having the centre of attraction in one of its
**foci**; and the motion takes place in accordance with Kepler's laws (see Astronomy). - 2 " In the same way we may conclude that in flat gratings any departure from a straight line has the effect of causing the dust in the slit and the spectrum to have different
**foci**- a fact sometimes observed " (Rowland, " On Concave Gratings for Optical Purposes," Phil. - If "=4), the term of the first order vanishes, and the reduction of the difference of path via P and via A to a term of the fourth order proves not only that Q and Q' are conjugate
**foci**, but also that the**foci**are exempt from the most important term in the aberration.

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