Fig. 183.—Red is represented by perpendicular lines; Green by oblique.
The following experiments will well serve to illustrate some of the more striking phenomena of double refraction, and will also be a useful introduction to its practical application. Take a plate of brass ([Fig. 183]) three inches by one, perforated with a series of holes from about one-sixteenth to one-fourth of an inch in diameter; the size of the smallest should be in accordance with the power of the objective, and the separating power of the double refraction.
Experiment 1.—Place the brass plate so that the smallest hole shall be in the centre of the stage of the microscope; employ a low power (1½ or 2 inches) objective, and adjust the focus as for the ordinary microscopic object; place the double image prism over the eye-piece, and two distinct images will be seen; by revolving the prism, the images will describe a circle, the circumference of which will cut the centre of the field of view; one of which is the ordinary, the other the extraordinary ray. By moving the slide from left to right the larger orifices will appear in the field, the images seen will not be completely separated, but will overlap, as represented in the figure.
Experiment 2.—Insert the Nicol’s prism into its place under the stage, still retaining the double image prism over the eye-piece; then, by examining the object, there will appear in some positions two images, in others only one image; it will be seen, that at 90° this ray will be cut off, and that which was first observed will become visible; at 180°, or one-half the circle, an alternate change will take place; at 270°, another change; and at 360°, the completion of the circle, the first image will reappear.
Before proceeding to make the next experiment, the position of the Nicol’s prism should be adjusted, and its angles brought parallel with the square of the stage. The true relative position of the selenite should also be determined by noticing the natural flaws in the film, which should run parallel with each other, and be adjusted at an angle of about 46° with the square bars of the stage.
Experiment 3.—If we now take the plate of selenite thus prepared, and place it under the piece of brass on the stage, we shall see, instead of the alternate black and white images, two coloured images composed of the constituents of white light, which will alternately change by revolving the eye-piece at every quarter of the circle; then, by passing along the brass, the images will overlap; and at the point at which they do so, white light will be produced. If, by accident, the prism be placed at an angle of 45° from the square part of the stage, no particular colour will be perceived, and it will then illustrate the phenomena of the neutral axis of the selenite, because when placed in the relative position no depolarisation takes place. The phenomena of polarised light may be further illustrated by the addition of a second double image prism, and a film of selenite adapted between the two. The systems of coloured rings in crystals cut perpendicularly to the principal axis of the crystal are best seen by employing the lowest object-glass.
Biaxial Crystals.—To show perfectly the beautiful series of rings and brushes which biaxial crystals exhibit, it becomes necessary to convert the microscope, for the time being, into (so to speak) a wide-angled telescope.
Huyghenian Eye-piece.