Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
fcion two images of the jar, formed by rays at right angles 
with those direct from the jar to the spectators. The great 
advantage of this method is, that the spectators only need 
to be about the same height as the jar, to receive perfectly 
polarised light all round; and the complementary effect 
can always be seen at the same moment in the mirrors, so 
that when the jar extinguishes direct light, the mirror 
images are bright. This having been demonstrated, we 
finally cover the top of the jar with a large quartz plate q. 
The jar itself will now glow with all the rotational colours 
as the polariser is rotated; differently-coloured images 
being seen at the same time in the mirrors m, and the 
whole forming a demonstration of indescribably delicate 
beauty. It is convenient to have the mirrors hinged 
together like a book, and to stand both jar and mirrors on 
a circular piece of board, if no higher support be needed, in 
order that the whole may be turned to face in succession all 
sides of the room. 
A NUMBEE of experiments showing the effects, nature, and 
qualitative relations of heat are easily capable of projection. 
Some specimen examples may suffice. 
224. Expansion.—As regards the expansion of solids, 
Gravesande’s ring and the various socket forms of apparatus 
need no remark. Any ordinary form of pyrometer is readily 
projected in action by the shadow method (§ 109) ; or a small


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