▁ir is ▁( dia ph rag m ) ▁in ▁a ▁camera ▁l ens ▁to ▁reduce ▁the ▁light ▁intensity . ▁ ▁For ▁relative ▁to ▁human ▁phys i ology , ▁foc using ▁on ▁a ▁bright ▁spot ▁of ▁light ▁such ▁as ▁a ▁pin ▁point ▁image ▁of ▁the ▁Sun ▁for ▁any ▁extended ▁period ▁or ▁repet it ively ▁over ▁a ▁short ▁duration ▁of ▁time ▁advers ely ▁effects ▁moment arily , ▁a ▁person ' s ▁visual ▁ac u ity , ▁thus ▁making ▁it ▁more ▁challeng ing ▁to ▁focus ▁one ' s ▁eyes ▁to ▁read ▁the ▁angular ▁scale . ▁It ▁being ▁typical ▁to ▁orient ate ▁the ▁instrument ▁such ▁that ▁the ▁operator ▁faces ▁looking ▁slightly ▁down ▁upon ▁the ▁scale , ▁the ▁Sun ▁at ▁the ▁users ▁left , ▁with ▁right ▁hand ▁placed ▁in ▁such ▁a ▁way ▁that ▁the ▁ray s ▁of ▁sun light ▁pass ▁through ▁the ▁two , ▁per for ated ▁sight ing ▁pl ates , ▁forming ▁a ▁bright ▁ill umin ated ▁spot ▁on ▁the ▁observer ' s ▁finger ▁( see ▁photo ), ▁the ▁finger ▁function ing ▁as ▁a ▁projection ▁screen . ▁ ▁At ▁the ▁moment ▁an ▁optical ▁alignment ▁with ▁the ▁Sun ▁is ▁established , ▁the ▁angular ▁value ▁of ▁the ▁device ▁is ▁read ▁by ▁the ▁operator ▁at ▁the ▁point ▁where ▁the ▁graduated ▁scale ▁is ▁bis ected ▁by ▁the ▁h anging ▁pl umb ▁line . ▁ ▁These ▁instruments , ▁with ▁poor ▁angular ▁resolution , ▁not ▁princip ally ▁intended ▁to ▁function ▁with ▁stars ▁at ▁night , ▁as ▁an ▁astronom ical ▁meas uring ▁device , ▁since ▁it ▁is ▁imp ract ical ▁to ▁sight ▁a ▁star