132 



XXI. — The Annular Condenser for the Microscope, a New Instru- 

 ment, Contrived and Described by George Shadbolt, Esq. 



(Read June 26, 1850.) 



The attention of the members of this Society having been several 

 times of late directed to the consideration of oblique illumination, I 

 am desirous of submitting on this, the last occasion before the recess, 

 a description of a new instrument I have contrived for the above- 

 mentioned purpose. I would premise that it is not intended to 

 supersede anj r apparatus at present in use, but simply to be made 

 use of as an additional tool for facilitating the acquisition of a know- 

 ledge of the structure of some classes of objects. I think it requisite 

 to make this statement, as a notion appears to exist amongst some 

 of our members that the various pieces of apparatus lately described 

 are designed by their advocates to displace others in all cases, 

 whereas this is very far from being the case. 



To a gentleman who has been very recently elected a member of 

 the Microscopical Society, Mr. Wenham, we are indebted for the idea 

 of throwing the light obliquely from every azimuth of the circle at 

 the same time, as illustrated in his parabolic condenser ; and it was 

 from viewing the effects produced by that instrument that I was led 

 to devise the one I am about to describe. In remarking upon what I 

 may consider slight disadvantages in the parabolic condenser, I cannot 

 too strongly disavow any intentional spirit of detraction ; for, indeed, I 

 entertain a very highly favorable opinion of its utility ; this, however, 

 should not prevent any attempts at simplification or improvement, 

 or deter from the endeavour to effect a similar object by other means. 

 In examining the parabolic condenser, there were three points 

 which struck me as being worth an effort to amend. In the first 

 place, all the rays do not fall on the object at the same angle. Se- 

 condly, the most valuable part of the parabola is necessarily cut off, 

 and the rays that are reflected from the remaining portion can none 

 of them be incident upon it at an angle of less than fifty degrees from 

 the perpendicular ; consequently a very large amount of the incident 

 light is lost without reflection. Thirdly, the construction of a para- 

 bola correct in figure is by no means an easy operation. 



To obviate these objections was my intention in devising the 



