302 S. C. AKEHURST ON SOME OBSERVATIONS CONCERNING 



improvement in the resulting definition." Mr. J. W. Gordon's 

 opinion is that when a suitable stop is employed in the sub-stage 

 condenser there is no objection to using annular light with an oil- 

 imniersion objective of high numerical aperture. It is, however, 

 necessary that the outer zones of the objective used should be 

 well corrected. His own method of blocking out the central 

 beam is to use a stop over the eye-piece and this is fully described 

 in the Journal R. M. S., February 1907. On the other hand, 

 Carpenter does not entirely agree that annular light is permissible. 

 Quoting from the seventh edition, he says, " If it is required to 

 accentuate a known structure, such as the perforated membrane of 

 a diatom, it can be done by annular illumination, which means 

 the same arrangement as for dark-ground but with a stop insuffi- 

 ciently large to shut out all the light. This method is not to be 

 recommended when a structure is unknown, as it is also liable to 

 give false images." 



Mr. Nelson has also expressed himself against annular light, 

 stating that whilst strong resolution of diatoms is obtained by 

 this method of illumination it also gives rise to spurious images. 



The subject of sub-stage illumination is a large one, and I am 

 only dealing with one phase of it, viz. annular light produced by 

 a reflecting condenser, to be used in conjunction with an oil- 

 immersion objective, for resolving the fine structure of diatoms 

 and displaying stained bacteria.* When a wide-angle refracting 

 condenser is employed, with stop to produce annular light, trouble 

 arises through chromatic aberration, which is especially noticeable 

 when an objective of high aperture is used. This dispersed colour 

 is objectionable, as it operates against a pure image being formed, 

 and is also detrimental to obtaining faithful records by photo- 

 graphy. Much has been undertaken to demonstrate that, in 

 practice, light from a condenser exhibiting chromatic aberration 



* A slide of Tubercle bacilli was exhibited illuminated with annular 

 light, showing that the reflecting condenser works well with small stained 

 objects in addition to diatoms. 



