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46
2.GrowthofABO
3andBO
2Crystals
ofthespecimenasa‘phaseobject’.Onthebasisofthemodeloftheweak
phase-objectapproximation,theobjectivelenstransferfunctionT(u)isde-
rived.IfthevalueofT(u)isnegative,thepositivephasecontrastisobtained
andtheimageofatomsisdarkagainstthelightbackground,whileifT(u)
ispositive,thenegativephase