For the draw-tube a 2-inch objective and a B Huyghenian eye-piece answers very well. Before screwing the objective on to the end of the draw-tube centre the light in the usual manner, the Nicol’s being turned so as to give a light field, then screw the objective on to the end of the aperture, and put the crystal on the stage, rack down the body so that the objective on the nose-piece nearly touches the crystal, then focus with the draw-tube only. The sub-stage condenser should be racked up close to the underside of the crystal. Opticians, however, have more recently furnished a special form of microscope (_The Petrological Microscope_, Fig. 79, p. 112), for the use of those students who may desire to prosecute so fascinating a study, and determine the optic axial angles of crystals. 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.