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Show Ill(ll I'rlra fur Nlral-litnraa. One of tbo dllllcult problems In practical prac-tical mechanical Is to make a "straightedge," "straight-edge," How difficult It Is may be Judged from un Incident which occurred oc-curred In tho thopa of J. A. Ilrashear. the astronomical Inainimeat-maker A customer nikwl Mr. Ilrashear what would bo tho price of "a perfect straight-edge ot glass thirty-six Inches lonte," "It can't be mado nbsolutely perfttt," said Mr. Ilrnshear, "but It could probably bo made with a limit cjf error amounting to only n fraction of n wavolcngth of light." "How mueJt would that cost?" "About forty thousand dollars'" It turned out that the euitomer wanted the strnlght-edgs for t semper, and that an error of ono sixty-fourth of an Inch would have been Insenilble for his purpose. |