Fig. 108.—Projection Eye-piece.
Schmidt’s goniometer positive eye-piece, for measuring the angles of crystals, is so arranged as to be easily rotated within a large and accurately graduated circle. In the focus of the eye-piece a single cobweb is drawn across, and to the upper part is attached a vernier. The crystals being placed in the field of the microscope, care being taken that they lie perfectly flat, the vernier is brought to zero, and then the whole apparatus turned until the line is parallel with one face of the crystal; the frame-work bearing the cobweb, with the vernier, is now rotated until the cobweb becomes parallel with the next face of the crystal, and the number of degrees which it has traversed may then be accurately read off.
Goniometer.—If a higher degree of precision is required, then, the double-refracting goniometer invented by the late Dr. Leeson must be substituted. With this goniometer ([Fig. 109]) the angles of crystals, whether microscopic or otherwise, can be measured. It has removed the earlier difficulties incident to similar instruments formerly in use. Among other advantages, it is capable of measuring opaque and even imperfect crystals, beside microscopic crystals and those in the interior of other transparent media. It is equally applicable to the largest crystals, and will measure angles without removing the crystal from a specimen, provided only the whole is placed on a suitable adjusting stage. The value of the goniometer depends on the application of a doubly refracting prism, either of Iceland spar or of quartz, cut of such a thickness as will partially separate the two images of the angle it is proposed to measure.
Dr. Leeson strongly insisted on the importance of the microscope in the examination of the planes of crystals subjected to measurement, as obliquity in many cases arises from not only conchoidal fractures, but also from imperfect laminæ elevating one portion of a plane, and yet allowing a very tolerable reflection when measured by the double refracting goniometer.
Fig. 109.—Leeson’s Goniometer.
Microscopes for crystallographic and petrological research are now specially constructed for measuring the angles of crystals.
Erector eye-pieces and erecting prisms are employed for the purpose of causing the image presented to the eye to correspond with that of the object. They are also helpful in making minute dissections of structure; the loss of light, however, by sending it through two additional surfaces is a drawback, and impairs the sharpness of the image. Nachet designed an extremely ingenious arrangement whereby the inverted image became erect; he adapted a simple rectangular prism to the eye-piece. The obliquity which a prism gives to the visual rays when the microscope is used in the erect position, as for dissecting, is an advantage, as it brings the image to the eye at an angle very nearly corresponding to that of the inclined position in which the microscope is ordinarily used.
The Achromatic Objective.
