SUB-STAGE ILLUMINATION. 305 



and for the photographs which illustrate this paper ; and you 

 will agree with me that without these records my remarks con- 

 cerning the value of reflected annular illumination would have 

 been much less convincing. 



Summary of the Advantages in using Annular Light 

 produced by reflecting condenser. 



(1) When employing an achromatic condenser excess of light 

 is reduced by closing the iris diaphragm. This involves a sacrifice 

 of the numerical aperture, and, therefore, loss of resolution. 

 With the reflecting concentric condenser there is no loss of high- 

 angle rays, the excess of light being modified by stopping out a 

 portion of the central or dioptric beam; the fullest possible 

 advantage can, therefore, be taken of the numerical aperture of 

 the whole optical system. 



(2) Chromatic dispersion being entirely eliminated, a pure 

 image is obtained. 



(3) The absence of colour in the field admits of critical work 

 being done by photo-micrography. 



(4) When necessity arises to search a slide for minute striae, 

 or other fine structure, it is immaterial in which direction across 

 the field the striae appear they are resolved. 



(5) The simple construction of this type of condenser admits 

 of it being produced at about half the cost of an achromatic oil- 

 immersion condenser ; and whilst it can only be employed with a 

 Y2-th inch oil-immersion objective in the manner already described, 

 yet it gives excellent dark-ground effects with all powers from 

 Ygth to ^th inch. 



One defect if defect it can be called is that, in its present 

 form, there is no method of controlling the light passed by altering 

 the size of the stop. It is just possible means can be devised to 

 allow of this being done. 



In my opinion there appears to be room for a reflecting con- 



