. Appleton's dictionary of machines, mechanics, engine-work, and engineering. ?*•#/ » j * r r 11 * urn. The central disk of the lens, which is employed in lights of the first order, and whoso focal distance- is 920 millimetres, or 3622 inches, is about 11 inches in diameter; and the annular rings which surround it gradually decrease in breadth, as they recede from the axis, from 2| to 1$ inches. The breadth ot any zone or ring is, within certain limits, a matter of choice, it being desirable, however, that no Dart of Vol. II—31) a y 610 SEA-LIGHTS. the lens should be much thicker than the rest
. Appleton's dictionary of machines, mechanics, engine-work, and engineering. ?*•#/ » j * r r 11 * urn. The central disk of the lens, which is employed in lights of the first order, and whoso focal distance- is 920 millimetres, or 3622 inches, is about 11 inches in diameter; and the annular rings which surround it gradually decrease in breadth, as they recede from the axis, from 2| to 1$ inches. The breadth ot any zone or ring is, within certain limits, a matter of choice, it being desirable, however, that no Dart of Vol. II—31) a y 610 SEA-LIGHTS. the lens should be much thicker than the rest, as well for the purpose of avoiding inconvenient projec-tions on its surface, as to permit the rays to pass through every part of it with nearly equal loss byabsorption. The objects to be attained in the polyzonal or cdmpound lens are chiefly, as above noticed,to correct the excessive aberration produced by refraction through a hemisphere or great segment, whoseedge would make the parallel rays falling on its curve surface converge to a point much nearer the lensthan the principal focus, as det
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Keywords: ., bookcentury1800, booksubjectmechanicalengineering, bookyear1861
