Fuess[32] lately introduced a new form of microscope for polarising and viewing biaxial crystals, which he believes to be needed, as in the ordinary microscope the opening of the polariser is scarcely a third of that of the condenser; moreover, it is not absolutely necessary that the polariser and analyser should be Nicol’s prisms. This fact was discovered by myself many years ago. Fuess utilises a bundle of thin glass plates, as in the older Nuremberg polariscope. The frame holding plates can be readily adjusted at the proper polarising angle, the analyser being the ordinary small Nicol, screwed above the objective. The illuminator is an Abbe’s triple condenser, of numerical aperture 1·40, which can be adjusted in the ordinary way. The front lens of this should have a diameter of 11·12 mm. and the lower lens of 30 mm. This increase in the condenser fully compensates for the loss of light by the bundle of glass plates, and also enables thick sections of crystals to be examined in convergent polarised light. The ocular used should have a large field; the A Huyghenian answers best. A suggestion to return to the original Nuremberg polariser is very opportune, as Iceland spar is becoming scarce.
Mr. A. Mickel accidentally discovered that an opalescent mirror can be converted into an excellent and inexpensive substitute for the Nicol-prism polariser.
Rotation of Plane of Polarisation.
When a plate of quartz (rock-crystal), even of considerable thickness, cut perpendicular to the axis, is interposed between the polariser and analyser, colour is exhibited, the tints changing as the analyser is rotated; and similar effects of colour are produced by employing, instead of quartz, a solution of sugar enclosed in a tube with plain glass ends.
The action thus exerted by quartz and sugar is called rotation of the plane of polarisation, a name which sufficiently expresses the observed phenomena. In the case of ordinary quartz, and solutions of sugar-candy, it is necessary to rotate the analyser in the direction of watch-hands as seen by the observer, and the rotation of the plane of polarisation is said to be right-handed. In the case of what is called left-handed quartz, and of solutions of non-crystallisable sugar, the rotation of the plane of polarisation is in the opposite direction, and the observer must rotate the analyser against watch-hands.
Quartz belongs to the uniaxial system of crystals, and accordingly exhibits one series of rings only, and no perfect central black cross.
On revolving the tourmaline the colour gradually changes, and passes through all the colours of the spectrum. It can be cut to exhibit either right-handed polarisation or left-handed polarisation and also to exhibit straight lines.
Calc Spar.—A uniaxial crystal showing only one system of rings, and a black cross, changing into a white cross on revolving the tourmaline.
Topaz.—A biaxial crystal exhibiting only one system of rings with one fringe, owing to the wide separation of the axes. The fringe and colours change on revolving the tourmaline.
Borax.—A biaxial crystal; the colours are seen to be more intense than in topaz, but the rings not so complete—only one set of rings can be seen, owing to their wide separation.