APERTOMETER (Pr., Ouverture mare ; Ger., Oeflnungmesser) An instrument for measuring the numerical aperture of a lens or objective. Of those forms used in photomicrography, the best was devised by Abbe, consisting of a semicircular glass plate with the various apertures figured on the outer edge. The straight edge of the glass is bevelled to an angle of 45°, and in the centre of the bevel is a metal disc pierced with a small hole. The lamp must be placed opposite and in line with this hole. The objective to be tested is focused on the metal edge of the hole, the draw-tube is removed, and a low-power objective, which forms part of the apparatus, is screwed into the lower end. The draw-tube with this second objective is replaced, and the auxiliary lens is focused by means of the draw - tube upon the back of the objective which is being tested, by sliding the draw-tube up and down till the images of two metal pointers on the outer edge of the apertometer are sharply defined. A band of
light should be seen across the field, and the pointers are moved till they just reach the edge of the luminous band, where it disappears from the field of view. The readings given by the outer edges of the metal pointers are added together, and the half of the sum gives the numerical aperture.
Cheshire's is an inexpensive and fairly accur ate apertometer, consisting of a glass disc with concentric rings on the under surface. The objective is focused on a mark in the centre of the plate, and the eyepiece is then removed. The observer looks down the tube and notes the number of rings which are visible on the back lens of the objective. The value of each ring is o•I numerical aperture (N.A.), and the total gives the N.A. of the objective.