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

12Ireadily splits up by cleavage) a sheet of paper with a small hole madewith a needle, placed behind the spar, then viewed against the light,it will be found that instead of one light we shall see apparentlytwo, andif the hole is sufficiently small, the holes appear entirely detached fromeach other, and upon revolving the spar one of the images will be seento revolve around the other which remains stationary, the latter is calledthe ordinary ray and the moving spot the extraordinary image. Theseparation of the two images depends upon the angle through whichthe crystal has been turned, when this amounts to a right angle, theseparation is at its greatest, and if the crystal be still further turned, theimages begin to come together again until another right angle iscompleted, when they superimpose.On this principal polarizing and analyzing instruments have beenconstructed by various combinations of wedges and prisms of Icelandspar.Before going further it is as well to state that the price of Icelandspar, has gone up in bounds, as there is no means of supplementingthe stock already in the market, as the mine from which it was obtainedis filled up by volcanic coal, and all the later specimens that havebeen placed upon the market, show, owing to the internal workings ofnature, that the crystals are nearly all imperfect, this having beingeffected by the violent convulsions, it is now nearly impossible to obtaina piece of spar that will enable the optician to cut a prism of muchover i^^ face, a comparatively small size.^\- 57-