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XII. — The Measurement of Working Aperture. 



By Hamilton Haetridge, M.A. 

 Fellow of King's College, Cambridge. 



{Bead June 18, 1913.) 



In all branches of microscopic research, the ability to measure 

 with accuracy the working and total apertures of the objective 

 lens-system is becoming of the greatest importance, since these 

 two factors set the limitinsr closeness that can exist between 

 elements of structure for resolution to take place. 



The method to be described is of wide application, since it can 

 also be adapted to the estimation of aplanatic aperture, and of the 

 anc^le of diffraction or refraction of a beam of lisrht incident on an 

 object on the stage. The importance of the latter lies in the fact 

 that from it the fineness of the structure can be calculated. 



The technique of the method is as follows. On the stage of the 

 Microscope is placed an Abbe apertometer plate, for the ordinary 

 eye-piece being substituted a special micrometer eye-piece, bearing 

 at its lower end a low-power objective. By this arrangement the 

 magnified image of the upper focal plane of the objective could be 

 observed and measured. The cursors are now placed in position on 

 the apertometer, opposite a given scale-division, and the cross line 

 of the micrometer set at the same time at the edge of the image oi 

 the cursors, as seen in the upper focal plane of the objective. 



The cursors are then set to another N.A. scale-division of the 

 plate, and the readings of the micrometer recorded. A number of 

 readings having been taken in this way, the points are plotted 

 on squared paper — the curve obtained giving the value of the 

 micrometer-scale in terms of the N.A. But it should be noted 

 that the curve thus obtained should approximate closely to a 

 straight line, but is only correct for the particular objective 

 with which it was obtained. This is, as a rule, of little moment, 

 because one or two standardized objectives are usually found 

 sufficient for most of the measurements met with in practice. 



Having obtained a calibration curve as above, the method of 

 finding the W.A. is as follows. Eeplace the ordinary eye-piece by 

 the one described above, and then set the cross line of the micro- 

 meter first on one and then on the other edge of the image of the 

 condenser diaphragm. These two readings are then obtained in 

 terms of N.A. by reference to the calibration curve. The great 



