▁limited ▁approximately ▁by ▁the ▁w avel ength ▁of ▁the ▁light ▁used ▁in ▁imag ing ▁or ▁a ▁few ▁hundred ▁nan om eters ▁for ▁visible ▁light ▁micro scop es . ▁Develop ments ▁in ▁ult rav io let ▁( U V ) ▁micro scop es , ▁led ▁by ▁Kö h ler ▁and ▁Ro hr , ▁increased ▁resol ving ▁power ▁by ▁a ▁factor ▁of ▁two . ▁However ▁this ▁required ▁expensive ▁quart z ▁opt ics , ▁due ▁to ▁the ▁absor ption ▁of ▁U V ▁by ▁glass . ▁It ▁was ▁believed ▁that ▁obtain ing ▁an ▁image ▁with ▁sub - mic rom eter ▁information ▁was ▁not ▁possible ▁due ▁to ▁this ▁w avel ength ▁constraint . ▁ ▁In ▁ 1 8 5 8 , ▁Pl ück er ▁observed ▁the ▁def lection ▁of ▁" c ath ode ▁ray s " ▁( elect rons ) ▁by ▁magnetic ▁fields . ▁This ▁effect ▁was ▁used ▁by ▁Ferdinand ▁Braun ▁in ▁ 1 8 9 7 ▁to ▁build ▁simple ▁c ath ode - ray ▁oscill os cope ▁( C RO ) ▁meas uring ▁devices . ▁In ▁ 1 8 9 1 , ▁R ie cke ▁noticed ▁that ▁the ▁c ath ode ▁ray s ▁could ▁be ▁focused ▁by ▁magnetic ▁fields , ▁allowing ▁for ▁simple ▁elect romagnet ic ▁l ens ▁designs . ▁In ▁ 1 9 2 6 , ▁Hans ▁Bus ch ▁published ▁work ▁extending ▁this ▁theory ▁and ▁showed ▁that ▁the ▁l ens ▁mak er ' s ▁equation ▁could , ▁with ▁appropriate ▁assumptions , ▁be ▁applied ▁to ▁electrons . ▁ ▁In ▁ 1 9 2 8 , ▁at ▁the