DIFFERENCEINGLORY 193"If thedirectlightofthe sunbe receivedupona
plateofpolishedblackglass,itcanbereflectedinany
directionuponthewallsofaroom. Thecharacterof
lightthusreflectedisradicallychanged. Theproper-
ties of the reflected ray are not now symmetrical
aroundtheray. Therearecertaindirectionsinwhich
itcannotagainbereflected."—Appleton's''Physics/'
Lightcomingfrom thesuncanbereflectedinany
directionbyamirrororbyaplateofpolishedblack
glass. Thisisanessentialof lightcomingfromthe
sun,or,forthatmatter,fromanyself-luminousbody
—thelightcanbereflectedinanydirection.
Butlightthathasoncebeenreflectedcannotbeagain
reflectedin every direction. There is one plane in
whichitrefusestobeagainreflected. Itisthensaid
tobepolarized. Whentwomirrorsaresetsothatthe
lightofeachisincidentatanangleof54°35',nolight
willbereflectedfromthesecondmirror;ablackspot
will appearinthefieldof view. Ifthe mirrorsare
keptatthisangle,andtheupperoneisrevolvedabout
averticalaxis,thelightwill growbrighteruntilthe
mirrorhasturned90°;thenitgrows feeblecontinu-
ouslyuntilthemirror hasturnedanother90°, when
itisagainzvhollyextinguished.
To discover whether light is reflected, an instru-
mentcalledthepolarscopeisused. Bymeansofthis
instrument,wemayexamineagivenrayoflight,and
knowdefinitelywhethertherayhascomedirectfrom
someself-luminous body,orif ithasbeen reflected.
Applying this investigation to the light that comes
fromthemoon,wefindthat themoonshinesbyre-
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