v538 Transactions of the Society. 



correspondence with the optical centre of the glass disc. The 

 edges of the silvered strips lying under the cover slip should 

 therefore be sharply focused. 



The adjustment of tube length is necessary, because it is found 

 by experiment that aperture varies with tube length, particularly 

 with lenses of long focal length. 



Further, the focusing of the objective should be carried out by 

 rays corresponding to the peripheral zone of the objective only ; 

 since it is these rays which are concerned when the aperture is 

 measured, and since other rays, if spherical aberration be present, 

 will come to a different focus. This limitation to peripheral rays 

 may be performed by the employment of an annular post objective 

 stop. (A Travis expanding stop is admirable for this purpose). {8).* 

 A fijrther advantage would appear to be given by this method in 

 cases where residual splierical aberration exists in the outer zone, 

 since the same focusing adjustment would be employed in making 

 the measurement of N".A. as that used when structure of the 

 utmost fineness that the objective will resolve is under examina- 

 tion. The tube length being correctly adjusted, and the post 

 objective annular stop being in place, the objective is carefully 

 focused. The image will in general be a very poor one compared 

 to tliat usually obtained, because of the aberrations which are 

 usually found in the peripheral zones of most objectives. In spite 

 •of this, however, precision of focus is as a rule considerable, because 

 of the large angle between the rays forming the image. 



In the ordinary technique witli the Abbe plate when the 

 microscope has been focused the eye-piece is removed, and one or 

 other of two methods employed : — 



(A) A special objective (supplied with the Abbe apertometer) 

 is screwed to the lower end of the draw tube so that a magnified 

 image of the back lens of the objective is seen in the eye-piece. 

 {B) The eye is placed over the draw tube and the back lens 

 examined direct. Both methods are faulty. The first in that the 

 aperture attached to the brass tube fitted to the apertometer 

 objective can correspond to one tube length only, and this will 

 be, by accident alone, that for which the focus has been made 

 previously. The second in that again the correction for tube 

 length is ignored, for the iris of the eye is the limiting aperture of 

 the rays, and should therefore correspond with the plane of the 

 image formed by the objective. But owing to the eye being 

 within a convergent optical system formed by the cornea and 

 aqueous, its effective aperture is considerably above the anatomical 

 position of the iris, and therefore the eye should in general be 

 placed 2 to 5 c.cm. below the top of the draw tube in order that the 



* The italic figures within brackets refer to the Bibliography at end of the 

 paper. 



