155 



If the light be caused to fall with sufficient obliquity, what is 

 known as the " black field illumination " is produced ; but in this 

 case, the shadow resulting from the light in one direction only is a 

 positive detriment to distinct vision, and generally renders the ob- 

 ject confused and deceptive in appearance. 



In order to obviate this objectionable shadow, as well as to pro- 

 cure a more brilliant illumination, the parabolic condenser was pro- 

 jected by Mr. Wenham, to whom alone belongs the credit of having 

 suggested the use of oblique illumination in every azimuth, so as to 

 produce a black field ; and for a detailed description of whose appa- 

 ratus I must refer to his paper, already published in the ' Transac- 

 tions ' of this Society. 



The great advantage of a black field illumination, under the condi- 

 tions above described (that is, free from the objectionable shadow), 

 becomes eminently apparent, for as the only visible rays proceed di- 

 rectly from the object under examination, and the eye is not fatigued 

 or the attention distracted by any extraneous light, the effect produ- 

 ced is almost tantamount to an enlargement of the aperture of the 

 objective. 



Having carefully considered this happy idea, so ingeniously brought 

 to bear in aiding the researches of the microscopist, it occurred to 

 me that it might be possible to carry out the principle still further 

 than had as yet been done, and at the same time to obtain a better 

 reflecting surface, and also greater facility in the manufacture of the 

 necessary apparatus, by making it of glass instead of metal. I was 

 at once met by a difficulty in testing this notion, owing to the exist- 

 ing necessity, in polishing glass, of a double motion, such as is 

 effected in lenses by changing the axis. Moreover, to produce a 

 paraboloid correct in figure, is by no means a very easy task : I con- 

 sequently devised the annular condenser, a hasty description of which 

 I gave in June of last year. Since that time I have endeavoured to 

 simplify the arrangement by making use of spherical surfaces, hav- 

 ing all their centres in the axis of the instrument, but here again I 

 was baffled by the great amount of aberration. 



I have however at length succeeded in devising an arrangement 

 of spherical curves, one centre only being eccentric, which fully an- 

 swers my expectations, and is easy of manufacture. The spherical 

 aberration, although not quite destroyed, is but trifling, and does 

 not materially impair the efficacy of the instrument, which I have 

 named the sphcero- annular condenser. 



