212 



APERTOMETER. 



{APPENDIX. 



DETERMINATION OF THE APERTURE OF OBJECTIVES. 

 § 353. Determination of the Aperture of Objectives with an Apertometer. — Ex- 

 cellent directions for using the Abbe apertometer maybe found in the Jour. Roy. 

 Micr. Soc, 1878, p. 19, and 1880, p. 20; in Dippel, Zimmerman and Czapski. The 

 following directions are but slightly modified from Carpenter-Dallinger, pp 337- 

 338. 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. 165. As seen by this figure 



Fig. 165. Abbe Apertometer. 

 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 aperture 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 noiv be moved so that their edges just touch the periphery of the 

 back lefts. They act, as it were, as a diaphragm to cut the fan and reduce it, so 

 that its angle just equals the aperture 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 aperture of the objective. As the nu- 

 merical apertures of these arcs are engraved on the apertometer they can be read 

 off by inspection. Nevertheless a difficulty is experienced, from the fact that it is 

 not easy to determine the exact point at which the edge of the screen touches the 

 periphery of the back lens, or as we prefer lo designate it, the limit of aperture, 

 for curious as the expression may appear we have found at times that the back 

 lens of an objective is larger than the aperture of the objective requires. In that 

 case the edges of the screen refuse to touch the periphery." 



In determining the aperture of homogeneous immersion objectives the proper 

 immersion fluid should be used as in ordinary observation. So, also, with glycerin 

 ©r water immersion objectives. 



