154 



XXVIII. — Observations upon Oblique Illumination ; with a Descrip- 

 tion of the Author's Spharo-annular Condenser. By George 

 Shadbolt, Esq. 



(Read March 19, 1851). 



Although the subject of oblique illumination has been so fre- 

 quently before the Society of late, I am induced once again to call 

 attention to it, from a conviction that by its aid, if rightly applied in 

 examining the structure of a certain class of bodies, much valuable 

 information may be acquired, otherwise very difficult, if not impossi- 

 ble, of attainment. 



Having devoted much time and consideration to this mode of ma- 

 nipulation, I purpose defining the principles which I conceive should 

 be kept in view, in order to obtain the greatest amount of advantage 

 derivable from it. 



This subject may be best considered 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 familiar to microscopists ; the 

 latter having been but recently suggested, and carried out by Mr. 

 Wenham in his valuable parabolic condenser. 



As regards oblique illumination in one direction, the cleverly con- 

 structed prism of M. Nachet (a description of which I had the ho- 

 nour of offering on a former occasion) affords an infinitely superior 

 mode of application to the old method of turning the mirror on 

 one side, besides some other advantages needless to recapitulate at 

 present. 



But whatever may be the mode adopted for condensing the light 

 on one side of an object, the chief advantage obtained is a certain 

 amount of shadow resulting from any variations of surface, which 

 renders such variations more perceptible by the contrast thus af- 

 forded. A fact tending to support this view is, that in certain objects 

 (those, for instance, which have minute ridges on the surface) the 

 details become visible only when the light is caused to fall at right 

 angles, or nearly so, to the course of the stria. 



