. Edinburgh New Philosophical Journal. 308 Rev. E. Craig on the Measurement of the a half long, having one lens at each extremity; and the tuhe is smoothly and accurately fitted, not to screw, but to slip down into the tube of the object-glasses, Fig. The lower lens O' receives the image from the object-glasses, and this image is e.\amined by means of the upper lens O. In the focus of the upper lens O, and lying directly on the plane side of the lower lens O', which is plano-convex, I placed two fine silk threads, crossing each <)ther at right angles. The tube of the eye-piece is slippe
. Edinburgh New Philosophical Journal. 308 Rev. E. Craig on the Measurement of the a half long, having one lens at each extremity; and the tuhe is smoothly and accurately fitted, not to screw, but to slip down into the tube of the object-glasses, Fig. The lower lens O' receives the image from the object-glasses, and this image is e.\amined by means of the upper lens O. In the focus of the upper lens O, and lying directly on the plane side of the lower lens O', which is plano-convex, I placed two fine silk threads, crossing each <)ther at right angles. The tube of the eye-piece is slipped down, not screwed down, to its right position in the tube of the object-glasses. It can be turned, therefore, either way freely, without affecting the focal arrange- ment of the microscope, which is regulated by other means. And, by turning the eye-piece round, these fine threads, un- magnified, may be applied to the side of any angle of a highly magnified crystal lying on the field below, A brass circle C, about two inches and a half in diameter, with a divided scale on its outer edge, was made to screw fast on to the object-tube T, before the insertion of the eye-piece. And to the edge of the [eye-piece a small index I was fixed, so as to lie close on the circle C, and point to the graduated divi- sion. This index is not a simple point, but a vernier, for in- dicating minutes. To measure the angle of a crystal F, it is necessary to bring the point of the angle to the centre of the field of view, where the silk diameters cross each other; then, by turning the eye- piece round, to bring one of these silk lines to coincide with one side of the angle to be measured, and notice the point of the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original
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