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CONOSCOPIC OR INTERFERENCE FIGURES AS SEEN FROM SMITH & BECK ACCESSORIES OF THE 1860's.

IMAGES PRODUCED BY A SMITH & BECK MICRSCOPE

COUNTRY: ENGLAND

c. 1860's

INTERFERENCE OR CONOSCOPIC FIGURES:

The figures illustrated on this page were produced using equipment from a Smith & Beck Best No. 1 microscope equipped in the 1860's. The figure on the left is centered but the field of view is too narrow for the camera; the image on the right illustrates more of the rings and isogyres(black areas between the colored rings) to one side. In the mid-19th century, the figures were usually produced using standard accessories available for this and other instruments. This included a polarizer beneath the stage attached to the bottom of the substage ring, a low power objective, and an analyzer eyepiece cap (or tourmaline eyepiece cap) atop an eyepiece, with a special eyepiece adapter containing a mineral crystal in between the analyzer cap on top, and the usual eyepiece optics below. A condenser could be used above the polarizer to brighten and focus the image. This Smith & Beck Best No 1 microscope is setup in this fashion, including the condenser. In the Smith & Beck catalog of 1865, the mineral eyecap adapter was simply called a 'crystal mounted to show rings'. The phenomena now known as conoscopic or interference figures, were first described by David Brewster in his Treatise on New Philosophical Instruments... of 1813.

When the polarized light axis of the polarizer and analyzer are rotated 90 degrees from each other, and the adapter with the crystal in it is attached under the analyzer,the figure is seen against a black background. In modern microscopy, an alternative arrangement is used; the crystal is placed on the stage, the analyzer is in the body tube, (or above the objective on a special adapter), and a special lens(either a phase telescope or a Bertrand lens)is used to visualize the figure at the back focal plane of the objective. An excerpt describing the mid 19th century use of these accesories from Beck's 1865 Treatise is available on this site. For a further explanation of interference figures, see the Wikipedia page about interference figures.