SUB-STAGE ILLUMINATION. 303 



does not prevent good work being accomplished. On the other 

 hand, I believe the reduction of chromatic dispersion to a minimum 

 leads towards an ideal system for critical work. 



The question now arises, if annular light is employed with 

 objectives of high aperture, how is the trouble arising through 

 chromatic aberration to be avoided. I suggest reflected, instead 

 of refracted, light being used. I came to this conclusion after 

 makincr a number of observations with a Leitz concentric reflect- 

 ing condenser. This condenser has two reflecting surfaces, one 

 convex and the other concave, and, as the rays are brought to a 

 focus by reflection only, there is no chromatic dispersion, and 

 spherical aberration is reduced to a minimum. The elimination 

 of spherical aberration, however, is not a matter of importance. 

 This was pointed out to me by Mr. J. W. Gordon, who has very 

 generously allowed me to make use of his remarks on this point. 



He says: "Light from the periphery of the condenser may 

 exhibit defects due to spherical aberration. This light, on reach- 

 ing the object, sets up a new impulse, and the rays emerging from 

 the object, and travelling towards the eye, will, in any plane con- 

 jugate to the plane of the stage, appear free from the original 

 defects of spherical aberration just as if they had started from 

 an independent source. No false images would, therefore, arise 

 from this cause in the image plane when light is used from a 

 sub-stage condenser that has not been corrected for spherical 

 -aberration." It should be carefully noted that this reflecting con- 

 denser was produced to obtain dark-ground effects, and was never 

 intended to be used in the manner I have employed it that is, in 

 conjunction with a T V inch oil-immersion objective without a funnel 

 stop to reduce the IS". A. of the objective. In its present form the 

 reflecting condenser I have passes too much light. The results 

 obtained, however, were sufficiently striking to arrest attention 

 when resolving fine structure of various diatoms. The transverse 

 striae of Amphlpleura pellucida in monobromide of naphthalin 

 were displayed. In realgar the same details were strongly shown, 



