PROCEEDINGS OF THE SOCIETY. 751 



miked eye being about sixty seconds, we had here visibility of objects 

 measuring less than ^^^ part of the closest distance at which two such 

 objects could be seen separated or resolved. 



It seemed to him that the " delicate tracery " on diatoms which Mr. 

 Gordon was so proud of, would be recognised as spurious appearances of 

 the intercostal order, which disappeared when critical illumination was 

 employed. 



As regarded the question of annular apertures, these had been 

 shunned by astronomers because the assumed theoretical advantage was 

 not borne out in practice ; with the unbroken full aperture practically 

 all the light was concentrated in the central part of the spurious disk, 

 the outer rings being almost invisible ; with an annular aperture, on the 

 contrary, a great part of the light was transferred from the central disk 

 to the rings, which became so bright as to prevent any increase of 

 resolution from being obtained. 



It was also worth remembering that the complicated image of 

 PUurosigma angulatum, which was calculated as the theoretical result 

 of the co-operation of the six diffraction spectra, and which was realised 

 by Abbe and Stephenson, could only be obtained by the use of an 

 equivalent to Mr. Gordon's top-stop ; and the same was true of all the 

 most startling false images obtained with Abbe's diffraction plate. 



He could not help thinking that the latent power of the Microscope 

 which the top-stop developed was the already great power of the Micro- 

 scope to deceive. 



Mr. Maurice Blood was much interested in the appearance shown in 

 the photographs of the bright jelly ring round the bacilli when the top 

 stop was used, but thought it curious that the folds (creases) in the 

 blood film were surrounded by a similar appearance. 



He suggested that these experiments should be repeated with apo- 

 chromatic objectives, to make certain that the effects were not produced 

 by differences in the correction of the central and marginal zones of the 

 objective. He also thought it was not quite fair to compare a narrow 

 central beam with wide annular illumination. 



Mr. Gordon said he thanked all who had taken part in this discus- 

 sion, and, not least, those who had subjected his paper to a searching 

 criticism, because this was the only way in which the merits of a new 

 proposal could be tested. Mr. Rheinberg had said that a top stop was 

 equivalent to a stop placed in the upper focal plane of the objective. 

 That was roughly correct, but it must be taken subject to the criticism 

 which Mr. Beck had passed upon it, and subject also to this further 

 point of difference, which for practical purposes was important — namely, 

 that in the back focal plane of the objective you have to deal with 

 spherical wave-fronts, whereas in the Ramsden disk the wave-fronts are 

 plane wave-fronts. It follows that if a plate of glass is employed to 

 carry the stop, and is introduced into the back focal . plane, it will dis- 

 turb the corrections, but in the Ramsden disk it will disturb nothing. 

 In a finely adjusted instrument, therefore, a stop cannot be usefully 

 placed in the back focal plane, and the Ramsden disk is the only possible 

 position for it. But that was a point which did not enter, as he under- 

 stood it, into Mr. Rheinberg's argument, and he thought that, for the 



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