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VI. — On " Central" Light in Besolution. 

 By J. W. Stephenson, F.R.M.S., F.E.A.S. 



(^Bead ISth January, 1886.) 



It may perhaps not be inopportune to refer to the question of 

 resolution by " central " light, as distinguished from obhque illumi- 

 nation, as we have heard from time to time of certain feats having 

 been performed with " central " light which require explanation. 



It has been said, for instance, that Amphi'pleura i^ellucida has 

 been so resolved, a statement which, I submit with great respect, 

 being inconsistent with the Abbe (diffraction) theory of microscopic 

 vision, must necessarily be incorrect, although doubtless made in 

 the most perfect good faith. 



The reason why the supposed resolution of Atnphipleura by 

 "central" light is considered to be remarkable, depends upon 

 the fact, that according to the diffraction theory, the full aperture 

 of an objective can only be utilized ^Yhen the direct beam and 

 the diffraction beam or beams are seen at the extreme margin 

 of the back lens of the objective. The nearer these beams approach 

 each other, the smaller is the aperture made use of, so that when 

 the direct beam is confined to a small area round the centre of 

 the back lens, the aperture is reduced to one half. If then as 

 many lines to the inch could be resolved in the latter case, as 

 when the two beams are wide apart, the Abbe theory would fall to 

 the ground. 



The suggestion I have referred to arises, however, from some 

 misunderstanding of what is " central " light. 



The most elementary definition of a centre, is that it is a point 

 within a circle, from which all parts of the circumference are equi- 

 distant. It might therefore be contended on this definition, that 

 as a beam of light cannot be a point, strictly " central " light is 

 impossible ; but, as my object is to discuss the question practi- 

 cally, I should define " central " light as a beam whose axis 

 coincides with that of the objective, and is as narrow as possible, 

 consistently with sufficient illuminating power. 



However narrow this quasi central beam may be, the peripheral 

 portions must be strictly speaking more or less oblique ; but with 

 small obliquities, it may for all practical purposes be treated as 

 central. With every increase in its width, however, the obliquity 

 must also increase, so that when it becomes wide enough to fill the 

 whole of the back combination of the objective (the maximum 

 obliquity being attained), we have a beam which combines not only 

 strictly central light, but every other degree of obliquity from zero 

 upwards. 



The term " central " light is obviously erroneously apphed to 

 such a beam as this, although its axis coincides with the axis of 



