The Helmholtz Theory of the Microscope. By J. W. Gordon. 411 



in the object plane is the optical projection of that produced by 

 the Eamsden circle. If, then, the Kamsden circle be not larger 

 than the pupil of the observer's eye, the instrumental image of 

 the central point will be seen exactly as it really is. 



This implies, of course, that the entire beam which fills the 

 aperture of the objective is focussed in the image. The case, then, 

 of the oblique beam is somewhat different from that of the beam 

 transmitted along the optical axis. For, if the other apertures are 

 only just large enough to transmit the axial beam unmutilated, 

 they will necessarily cut down the oblique beams to less numerical 

 apertures than that of the axial beam and so impair the antipoint 

 in the marginal parts of the field of the instrument. Take, for 

 example, the oblique pencil from e^ It falls excentrically upon 

 the back lens of the objective, and consequently, if this back lens 

 has no greater aperture than the optical projection of the front 

 lens along the path of the axial beam, its edge will cut down the 

 aperture of this oblique beam all through the instrument. In 

 that case, the antipoint formed at rj x will be greater than the anti- 

 point at 77, and will be unsymmetrical. 

 For the effect of the cutting down of 

 the oblique beam by the two circular 

 apertures in succession will be to give 

 to the beam a sectional form as shown 

 in fig. 95. Its antipoint will have ap- 

 proximately the same shape, with the 

 long axis turned towards the centre of Fig. 95. 



the field. For this reason it is scarcely 



possible' to obtain, even with the most perfect optical arrangements, 

 a really good resolution in the peripheral parts of the field of the 

 Microscope. The aperture of the oblique beams which focus in 

 those regions has usually been cut down by diaphragms placed 

 either in front of or behind the principal diaphragm of the objective 

 to something less and often greatly less than the aperture of the 

 central beam. 



It is not only in the marginal regions of the instrumental field 

 that the deterioration due to this mutilation of the oblique beams 

 manifests itself. The distorted antipoints produced by this means 

 have a pernicious property of turning their long axes towards 

 the centre of the field, and consequently they tend to produce 

 their maximum disturbance there, where the resolution ought to be 

 finest. 



Fig. 96 illustrates this point by a diagram, in which five anti- 

 points are shown in their mutual relations. The four external ones 

 stand so far apart from the central one, that if they had the regular 

 size and form, similar to the size and form of the central antipoint, 

 they would not trench upon it at all. This is indicated by the 

 four circular outlines touching the middle circle. But assume 



