Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
ciably separated, owing to the thinness of the crystal. When 
these beams, polarised in rectangular planes, are again re¬ 
solved by the analyser, we have two similar sets of waves 
brought into interference, since one set has been retarded 
more than the other in the slice of crystal. If the retardation 
does not exceed a few wave-lengths, therefore, we get colour, 
and this is complementary in positions of the analyser differ¬ 
ing by 90°.1 
It is usual to show this by a selenite giving red and green 
images, and yellow and blue. The two may be mounted in 
one slide. That no interference is produced whilst the vibra¬ 
tions are in rectangular planes, is shown by removing the 
analyser. On adding this and rotating, the complementary 
colours appear. But when the analyser is in an intermediate 
position, no colour appears, one ray from the selenite being 
then suppressed by the analyser, and the other transmitted 
without modification. 
That the phenomena of interference are essentially similar 
to those of a thin film (only that in a selenite the retardation 
is that of the difference of two paths through the film, while 
in a ‘ thin film ’ one of the rays is retarded by twice the whole 
thickness, so that the selenite or mica is much thicker than 
a ‘ thin film ’ producing the same retardation), is best demon¬ 
strated by spectrum analysis, just as the Newton’s rings and 
soap film were analysed. A plate of crystal ground concave 
or convex exhibits Newton’s rings, which are readily analysed 
by placing a slit in a thin plate of metal across the slide in 
the stage, focussing, and interposing a prism (the compound 
direct prism is much the handiest for such experiments). The 
slit across a Newton’s ring slide shows precisely the same 
1 It must be understood that this work is solely concerned with practical 
experimental conditions. No attempt is made or intended to explain pheno¬ 
mena, or even to show their relations, further than the order of experiments may 
do so. For some elementary explanation the reader is referred to my treatise 
on Light : a Course of Experimental Optics, or other systematic treatises on 
the subject may be consulted. 


Sehr geehrte Benutzer,

aufgrund der aktuellen Entwicklungen in der Webtechnologie, die im Goobi viewer verwendet wird, unterstützt die Software den von Ihnen verwendeten Browser nicht mehr.

Bitte benutzen Sie einen der folgenden Browser, um diese Seite korrekt darstellen zu können.

Vielen Dank für Ihr Verständnis.