The Art of projection and complete magic lantern ... - Yesterday Image

120POLARIZEDLIGHT.Before attempting to explain the nature of polarised liglit, it may benecessary that we should define the title, and so answer the oft repeatedquestion :" What is polarised light ? " — It is light reflected from ortransmitted through glass at an angle of incidence of 56° 45^ ;accordingto the undidatory theory, light is propagated in two planes at least, atright angles to each other, whereas polarised light is propagated by onlyone plane separating as it were these rectangular planes, and in so doingacquires certain properties which were supposed to bear some relationto the opposite poles of a magnet, unfortunately the term " polarised "affords no indication of the phenomena itprofesses to describe.Some two centuries since, a traveller brought from Roerford somecrystals which eventually came into the hands of aa man of great learning, who upon examination made theDoctor Bartholinus,discovery thatall objects when viewed through the crystal appeared double, thisoccasioned no little surprise to the students of the theory of refractionwhich at the introduction of Iceland spar required some modification.Iceland spar, is also called calc spar, arising from its chemicalcomposition, carbonate of lime, and is sometimes termed rhombohedralspar, from the shape of itscrystals.The four principal processes by which light may be polarized, arereflection, ordinary refraction, double refraction and scattering by smallparticles, that alone which can be adapted to the use, and in connectionwith the lantern will be dealt with. If a piece of spar be taken, (whichFig- 56-A BLOCK OF Iceland spar, showing double refractions.{By kind permission of \l^i,ix