346 Transactions of the Society. 



With the microscope set up in the usual manner (the tube 

 length being correctly adjusted), and with the apertometer on the 

 stage, the annular stop is slipped into position behind the objective, 

 and the latter then carefully focused on one of the edges of the 

 silvered bands under the cover slip. The eye-piece is now re- 

 placed by the 2 mm. aperture parfocal with the eye-piece, which 

 is lit from above by a small frosted electric lamp of low candle- 

 power (any otlier suitable method of illumination may of course be 

 employed). 



The aperture being illuminated, the apertometer plate is gently 

 shifted so that the image of the aperture formed by the objective 

 shall fall on the centre of the cover. With a Eamsden eye-piece 

 of medium power (or a hand magnifier) the edge of the shadow on 

 the periphery of the apertometer plate is examined, and one of the 

 indices is moved, so that the upper prong of the trident is set to 

 the exact edge of the shadow. The lower edge is now examined, 

 and if found to be in exact contact the setting of the apertometer 

 plate is correct ; if not in contact the setting requires adjustment 

 as described in Section 5. When the correct adjustment has been 

 obtained, the central prong is placed in exact contact with the 

 shadow, and the scale-reading now taken. The left-hand index is 

 then adjusted and its reading similarly obtained. Ten alternate 

 readings are logged and a mean taken ; any considerable difference 

 between any of the sets of readings of the same side indicates that 

 the apparatus has been accidentally displaced, and the whole 

 process must be repeated. Assuming that the graduations of the 

 apertometer are accurate, the values obtained by this technique 

 may be depended on to 0-003 N.A. This error even may be 

 reduced by accurate apparatus. A method of this accuracy is 

 found to be essential if other apparatus is to be calibrated in terms 

 of N.A. 



Section 9. — Simplified Technique. 



Abbe has pointed out that for ordinary purposes very accurate 

 determinations of aperture are not required, because with most 

 objectives quite considerable variation in aperture is not found to 

 cause any observable change in resolving power. 



It is therefore possible in such cases to employ a simpler tech- 

 nique than that described above. 



For dry lenses of low and medium power the apertometer 

 described by Cheshire {5), or better, Ainslie's modification {6), can 

 be utilized. Special care should be taken to see that the tube 

 length is correct ; the use of an annular post objective stop is not 

 necessary however. But there are definite advantages from the use 

 of a descending light path, because the edge of the cone of divergent 

 rays is more easily observed, and the number of rings counted with 



