W. B. STOKES ON IMAGES OF DIATOM STRUCTURE. Ill 



There is, as the last method shows, an image for each zone of 

 the objective, and on examining the results we see that appa- 

 rently the " white dot " image is formed by the direct light, and 

 the " black dot " is formed by diffracted Hght. 



What would be the real effect of spherical aberration ? I have 

 been told that most objectives are spherically under-corrected. 

 If this be so, the foregoing suggestion is strongly supported : for 

 one must bring an under-corrected objective nearer to the object 

 in order to utilise the periphery of the lens. Compare this with 

 our three previous experiments, and agreement will be found. 



To return to our question: Which is the true image? Let 

 us deal with Mr. Nelson's deductions from the examination of 

 Pleurosigma formosum. This diatom differs from the majority 

 of diatoms in this respect : When an axial cone is employed the 

 "black dot" image is above the "white dot," the very reverse 

 of what occurs with the great majority of diatoms. This, in my 

 opinion, points to a difference or peculiarity in the structure of 

 P. formosum. 



I suggest that the perforations of this diatom are more or less 

 silted up wdth foreign material. When the late Mr. C. Haughton 

 Gill charged the minute perforations in diatom valves with 

 opaque metallic sulphides, the " white dot " image of the per- 

 forations was changed to a " black dot " and the '' black dot " to 

 a " white dot " ; but no such changes took place in the appear- 

 ances of P. foTTiiosur)!, the " black dot " image being merely 

 intensified. Does not this point to the probability of the cor- 

 rectness of my suggestion — viz. that the perforations of this 

 diatom were already filled up ? 



Again, the difficulty in getting an image of a ''postage stamp" 

 fracture through the perforations of this diatom was thought by 

 Mr. T. F. Smith to be due to the existence of a layer of beaded 

 structure, which would present the appearance of circles bulging 

 outwards ; but I imagine that the clinging of any silting-up 

 material to the broken-into openings would account satisfactorily 

 for such appearances. 



The minute appearances which Mr. Nelson discovered luithin 

 the black dots of this diatom may be considered, not as pieces of 

 silex or other material, but as irregular openings through the 

 silted-up perforations. They are bright at the upper focus and 

 dark at the lower focus when an axial cone is employed; that 



