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POPULAR SCIENCE REVIEW. 
striking phenomena in microscopical science of the present 
day. 
Whether this colour test, for which we are indebted to Dr. 
Piggott, is explained on the theory of the vibrating wave 
lengths corresponding to the infinitesimal thickness of films, as 
in the gorgeous colouring of the soap bubble, or is dependent 
upon a peculiar compounding of the colours by the glasses 
themselves, or upon the irradiation, refraction, and internal 
reflections of the spherical beads of which all scales and 
diatoms appear to be built up, are questions of so recondite a 
nature as to be worthy the consideration of the most advanced 
physicists of the day, and I hope Dr. Piggott will not be left 
alone in his glory, 
POLARISED LIGHT, 
Keturning for a moment to the equilateral prism, I will con- 
clude my suggestions for examining objects we can neither see 
nor touch, by recording the fact that the prism is an admirable 
■ polariser , While the light obtained at the angle of total 
reflection from an inner face of the prism is not perceptibly 
altered either in nature or quantity, yet the same incident 
pencil, if reflected from an outer face of the prism, supplies a 
purely polarised beam . As such, I have used it in the exami- 
nation of several suitable objects, passing it through selenite 
plates where necessary, and I prefer the results to any pre- 
viously obtained by the direct light of a common Nicol’s prism. 
Salicine and crystals generally, as well as fine vessels in animal 
and vegetable tissues, are seen in almost stereoscopic relief, in 
consequence of the shadows which are readily thrown by a slight 
obliquity of the polarising pencil. This obliquity may be 
extended to the bringing out the effect of polarised light, even 
on a dark ground, and thus, as in the combination devised by 
Mr. Furze, heightening the solidity by the play of colours. 
The prism when adapted to the microscope is mounted on a 
small arm with ball-and-socket joint, and, by varied adjust- 
ments of the prism, direct, oblique, dark-ground, and polarising 
illumination may be produced ; so that an object may in turn 
be investigated by every kind of illumination without change 
of apparatus or readjustment of the object. Mr. Highley, at my 
request, has brought out a popular instrument, in which the 
expensive luxuries of a mechanical sub-stage and elaborate con- 
densers are dispensed with, and his prism microscope , consist- 
ing of the body with its powers, a thin stage, and a two-inch 
equilateral prism looks like a good working tool, and cannot 
fail to interpret the minute wonders of creation to the intelli- 
gent admirer of nature. 
The following explanation is from the pen of Dr. Pigott, who 
kindly supplied the illustrations for this paper. 
