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March 19, 1851. Dv. Aitliur Fane, President, in the chair, 

 A paper by George Sliadbolt, Esq., entitled ' Observations upon 

 Oblique Illumination, with a description of the author's Sphaero- 

 annular Condenser,' was read. 



After some preliminary remarks, Mr. Shadbolt stated that the sub- 

 ject of oblique illumination might be considered as comprehended 

 under two distinct heads, viz., illumination by oblique light on one 

 side only, and illumination by opposing rays, so as to obviate any 

 shadow. The former mode has been long employed by microscopists, 

 but the latter has been suggested and carried out only recently, by 

 Mr. Wenham, in his parabolic condenser. The author considered 

 that by far the most advantageous mode of applying the first of these 

 methods, was by means of the cleverly-constructed prism of M. 

 Nachet, the effects of which are far superior to the old method of 

 turning the mirror on one side, and the instrument also possesses 

 some other obvious advantages. In this method of observing objects 

 the minute ridges are rendered clearly visible by means of their sha- 

 dows. But in order to view certain objects in the most advantageous 

 manner, it is desirable to get rid of the shadow entirely, and this, as 

 well as a far more brilliant illumination, is effected by the parabolic 

 condenser constructed by Mr. Wenham, which is fully described in a 

 late part of the Society's ' Transactions.' There are, however, certain 

 practical difficulties in constructing a paraboloid correctly, which ren- 

 der it by no means an easy task, and the author was in consequence 

 induced to devise his annular condenser, also described in a recent 

 part of the 'Transactions.' Still, however, the action was not perfectly 

 satisfactory ; and Mr. Shadbolt, after many trials, has succeeded in 

 producing an arrangement of sphaerical curves, one centre only being 

 excentric, which fully answers his expectations, and is easy of manu- 

 facture ; this he names the sph aero- annular condenser. It consists of 

 a portion cut off from a sphere of glass, the lower part being flat and 

 parallel to the object, and the upper surface concave. Its action is as 

 follows : — The light is reflected from the surface of the plane mirror 

 in parallel rays, which, falling perpendicularly on the base of the con- 

 denser, suffer no refraction, but pass on to the convex surface of the 

 sides, where, as the angle of incidence is in no case less than 45°, 

 they are totally reflected, and thus brought to a focus in the best 

 place for producing this kind of illumination. Diagrams explaining 

 the principles of the construction of this instrument, and the mode of 

 its action, were also exhibited and described. 



Vol. IV. . Y 



