Aji IfTiproved Method of Apertometry. 33 



eft'ective aperture should be in its correct position. ' In most micro- 

 scopes this position is impossible. Further, as a rule no care is 

 taken to keep the eye central with the draw tube, in order that its 

 effective aperture shall correspond with the centre of the field. 

 The correct tecimique is obtained by dropping into the draw tube 

 in place of the eye-piece a suitable aperture in a sunk brass mount, 

 the aperture being small and corresponding to the centre of the 

 field, the depth of the mount being such that the aperture is 

 parfocal with the eye-piece. 



Section 2. — Error in Apertometry due to the Use of 

 Eays not Corresponding to the Exact Centre of the 

 Field. 



Eays lying away from the centre give values which differ 

 considerably from those obtained with the employment of central 

 rays, because such rays pass through the apertometer plate as 

 bundles to one side of, and not necessarily parallel with, those which 

 illuminate the centre of the field. Since the definition of most 

 microscope objectives is superior at the centre of the field, obser- 

 vation is preferably made there, and therefore central rays should 

 be used for apertometry. There is, however, another and more 

 important reason. For accurate apertometry the indices and the 

 image of the restricting aperture of the objective should coincide. 

 This is rarely the case in practice, for with low-power objectives 

 the images of the indices are usually in a plane much above the 

 back lens ; whereas in the case of high-power lenses the plane is 

 situated more often within the objective. Further, while in most 

 objectives it is the back lens which limits the aperture, in one 

 objective in my possession this is done by the front lens, and there- 

 fore it is with the image of this that the indices should coincide. 

 As a rule, therefore, the image of the restricting aperture (usually 

 the back lens) of the objective and the indices do not coincide, and 

 therefore by focusing alone it is impossible to get both in focus at 

 the same time. 



Now, consideration showed that by restricting the effective iris 

 of the system it should be possible to increase the depth of focus 

 to such an extent that the images of the indices and the restrict- 

 ing aperture should be both simultaneously sharply imaged. It 

 was further seen that the effective iris corresponded in position 

 with the focused image of the ol.)jective. Therefore by replacing 

 the eye-piece by a small aperture, through which the observations 

 of N.A. could be made, it was possible to avoid the difficulty due 

 to the indices and the restricting aperture of the objective not 

 Coinciding. 



2 A 2 



