es ▁in ▁this ▁v at ... in ▁the ▁same ▁way , ▁all ▁of ▁the ▁parts ▁of ▁the ▁subt le ▁material , ▁which ▁are ▁touched ▁by ▁the ▁side ▁of ▁the ▁sun ▁that ▁faces ▁us , ▁tend ▁in ▁a ▁straight ▁line ▁towards ▁our ▁eyes ▁at ▁the ▁very ▁instant ▁that ▁we ▁open ▁them , ▁without ▁these ▁parts ▁imp eding ▁each ▁other , ▁and ▁even ▁without ▁their ▁being ▁imp eded ▁by ▁the ▁heav ier ▁particles ▁of ▁transparent ▁bodies ▁which ▁are ▁between ▁the ▁two . ▁ ▁Second ▁Disc ourse : ▁On ▁Ref raction ▁ ▁Des cart es ▁uses ▁a ▁tennis ▁ball ▁to ▁create ▁a ▁proof ▁for ▁the ▁laws ▁of ▁reflection ▁and ▁ref raction ▁in ▁his ▁third ▁model . ▁This ▁was ▁important ▁because ▁he ▁was ▁using ▁real - world ▁objects ▁( in ▁this ▁case , ▁a ▁tennis ▁ball ) ▁to ▁construct ▁mathematical ▁theory . ▁Des cart es ' ▁third ▁model ▁creates ▁a ▁mathematical ▁equation ▁for ▁the ▁Law ▁of ▁Ref raction , ▁character ized ▁by ▁the ▁angle ▁of ▁inc idence ▁equ alling ▁the ▁angle ▁of ▁ref raction . ▁In ▁today ' s ▁notation , ▁the ▁law ▁of ▁ref raction ▁states , ▁ ▁sin ▁i ▁= ▁n ▁sin ▁r , ▁where ▁i ▁is ▁the ▁angle ▁of ▁inc idence , ▁r ▁is ▁the ▁angle ▁of ▁ref raction , ▁and ▁n ▁is ▁the ▁index ▁of ▁ref raction . ▁Using ▁a ▁tennis ▁ball , ▁Des cart es ▁would ▁compare ▁the ▁projection ▁of ▁a ▁ray ▁of ▁light ▁to ▁the ▁way ▁a ▁ball ▁moves ▁when ▁it ▁is ▁thrown ▁up ▁against ▁another ▁object . ▁ ▁Cont ro