270 TEST PLATE AND APERTOMETER \CH. X 



portion (with good color correction in the middle) are caused by unequal magnifi- 

 cation of the different colored images." 



"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- 

 siderable." 



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, 1878, p. 19, and 1880, 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 \>y this figure 

 it consists of a semi-circular plate of glass. Along the straight edge or chord the 

 glass is beveled at 45 , 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 back lens will be seen stretched 

 across, as it were, the image of the circular part of the apertometer. It will ap- 

 pear as a bright band, because the light which enters normally at the surface is re- 

 flected by the beveled part of the chord in a vertical direction so that in reality a 

 fan of 180 in air is formed. There are two sliding screens seen on either side of 

 the apertometer ; they slide on the vertical circular portion of the instrument. 

 The images of these screens can be seen in the image of the bright band. These 

 screens should now be moved so that their edges just touch the periphery of the back 

 lens. They act, as it were, as a diaphragm to cut the fan and reduce it, so that its 

 angle just equals the aperature'of the objective and no more." "This angle is 

 now determined by the arc of glass between the screens ; thus we get an angle in 

 glass the exact equivalent of the aperature of the objective. As the numerical ap- 

 ertures of these arcs are engraved on the apertometer they can be read off by inspec- 

 tion. Nevertheless a difficulty is experienced, from the fact that it is not easy to 



