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



with directly transmitted light, one naturally expects a perfora- 

 tion to appear brighter than the substance surrounding it ; and 

 I would point out that opaque materials present in this image 

 the only appearance one would expect, viz. black. 



Mr. Nelson preferred the " black dot " image because he was 

 able to see fine differences and minute details in individual 

 *' black dots " of Phurosigma formosum, which were quite 

 invisible at any other focus. 



I do not know Mr. Comber's reasons for legarding the true 

 image to exist between the " black " and " white dot '' images. 



The true image, if it exists, must lie at the true focus. But 

 if we divide our objective into zones, may not spherical aberration 

 allow, within limits, a true focus to each zone ? 



Now, the conditions under which the " black " and " white dot " 

 images are seen as separate and distinct entities are those in 

 which the objective is divided into zones. 



When an axial cone is used the objective is divided into two 

 concentric zones, the central zone occupied mainly by directly 

 transmitted light, the peripheral zone occupied only by indirectly 

 transmitted (diffracted) light. 



An annular cone (full cone and a central stop in the condenser) 

 also divides the objective into two zones; but in this case the 

 diffracted light occupies the central, the directly transmitted 

 light the peripheral zone of the objective. 



A " half-way " annular cone (a two-thirds cone with a central 

 stop half the size of the diaphragm opening) divides the objective 

 into three zones, of which the central and peripheral zones are 

 occupi3d by diffracted light only, and the intermediate zone is 

 occupied by direct light. 



Examine an ordinary balsam-mounted diatom (not a Pleuro- 



siymct) by these three methods, and, when focussing down, you 



will find the positions of the "black dot" and "white dot" 



images to be as follows : 



. . 1 p' white dot." 



Axial cone ... ...- 



I" black dot." 



-n, ,1 1 /"'black dot." 



l!u]l annulus ... ..;- 



("white dot." 



"black dot." 

 Two thirds annulus ...J " white dot." 



r 



" black dot. 



