. The microscope; an introduction to microscopic methods and to histology. Microscopes. 270 PES T PLATE A NO A PER TO ME TER \_CH. X portion (with good color correction in the middle) are caused by unequal magnifi- cation of the different colored ; "Imperfections of this kind, improperly called "curvature of the field," are shown to a greater or less extent in the best objectives, where the aperture is con- ;. ii&*°°.. .".1615 *'-*%,>„ --Carl ZeLS^Apert^ejep Ienj^__ Fig. 216. Abbe Apertometer. \ 430. Determination of the Aperture of Objecti
. The microscope; an introduction to microscopic methods and to histology. Microscopes. 270 PES T PLATE A NO A PER TO ME TER \_CH. X portion (with good color correction in the middle) are caused by unequal magnifi- cation of the different colored ; "Imperfections of this kind, improperly called "curvature of the field," are shown to a greater or less extent in the best objectives, where the aperture is con- ;. ii&*°°.. .".1615 *'-*%,>„ --Carl ZeLS^Apert^ejep Ienj^__ Fig. 216. Abbe Apertometer. \ 430. Determination of the Aperture of Objectives with an Apertometer.— Excellent directions for using the Abbe apertometer may be found in the Jour. Roy. Micr. Soc, 1S78, p. 19, and 1S80, p. 20 ; in Dippel, Zimmermann and Czapski. The following directions are but slightly modified from Carpenter-Dallinger, pp. 394-396. The Abbe apertometer involves the same principle as that of Tolles, but it is carried out in a simpler manner ; it is shown in Fig. 216. As seen by this figure it consists of a semi-circular plate of glass. Along the straight edge or chord the glass is beveled at 450, and near this straight edge is a small, perforated circle, the perforation being in the center of the circle. To use the apertometer the micro- scope is placed in a vertical position, and the perforated circle is put under the mi- croscope and accurately focused. The circular edge of the apertometer is turned toward a window or plenty of artificial light so that the whole edge is lighted. When the objective is carefully focused on the perforated circle the draw-tube is removed and in its lower end is inserted the special objective which accompanies the apertometer. This objective and the ocular form a low power compound mi- croscope, and with it the back lens of the objective, whose aperature is to be meas- ured, is observed. The draw-tube is inserted and lowered until the back lens of the objective is in focus. "In the image of the
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