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See also:OBIT (through O. Fr., from See also:Lat. obitus, See also:death, obire, to go down, to See also:die) , a See also:term for See also:death, formerly used for the See also:account 2 The last See also:history of See also:Hugo of See also:Toul (12th See also:century) was the authority of Jacques de Guyse (14th century) in his Annales historiae princip . Hannoniae (Mon . Germ. See also:xxx.), where there is an account (bk. ix. ch . 6) of Alberich . of any curvature to within an See also:error of nth% of the See also:radius: male and See also:female templets being used for very deep curves, and the See also:spherometer for tools of longer radii (by appropriate grinding together, the radii are alterable at will within narrow, but sufficient, limits) . The accuracy attained in the grinding, however, is open to very perceptible modification by the subsequent polishing and figuring processes . This is particularly undesirable in the See also:case of deep curves and large apertures . A variation in a radius of curvature may occasion a little spherical See also:aberration at the axial See also:focus, but if the amount be small it may be neutralized by imparting to the See also:lens a parabolic See also:form or its opposite . Such an artifice is frequently adopted in correcting large See also:telescope objectives . With See also:optical systems which transmit large pencils with considerable obliquity (such as wide See also:angle photographic objectives) the curves are very deep, and a departure from the true radius which would be tolerated in a telescope cannot be permitted here . Such lenses are usually tested by means of a See also:master See also:curve worked in See also:glass . The master curve is fitted to the experimental lens, and an inspection of the interference fringes shows the quality of the See also:fit—whether it be perfect, or too shallow or too deep .
The workman then modifies his polisher or stroke in See also:order to correct the divergence
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See also:Flat surfaces are tested similarly
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This test by contact has been strongly advocated and has been regarded as sufficient to detect all irregularities of any moment
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This claim, however, is not justified, for the test is not sensitive to errors sufficient in amount to render a telescope See also:objective almost valueless; but such errors are easily discernible by other optical devices
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In See also:general, accuracy in the radii of curvature is of See also:primary importance and trueness of figuring is of secondary importance in photographic objectives, while the See also:reverse holds with telescopic objectives; in wide angle microscopic objectives these two conditions are of equal moment
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See also:Eye pieces do not require the same degree of accuracy either
of a See also:person's death (now " obituary ")
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An " See also:obit " was also a service performed at a funeral or in See also:commemoration of a dead person, particularly the founder or benefactor of a See also: |
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