E. M. NELSON ON THE BLACK AND WHITE DOT. 263 



appearing in the perforations of the diatom instead of the bright 

 disc at (a). A smaller-sized perforation will give an image like 

 (c), and the jY. lyra type the well-known images like (d) and (e). 

 This illustrates precisely what Mr. Stokes calls " edges and 

 perforations," (a) showing a large perforation with a black edge, 

 (c) a smaller one, and at (d) the edges have met. There is 

 no doubt that Mr. Stokes is right in assigning to si^herical 

 aberration an important function, for in those cases where there 

 is a white dot both above and below the black dot, the upper 

 or lower white dot can be made the stronger or weaker, or 

 both made alike, by means of screw-collar or tube -length 

 adjustments. 



Some experiments lately made have given me reason to 

 believe that the upper black dots which Mr. Stokes observed 

 when he employed annular illumination must have been images 

 of the stop at the back of the condenser ; if the arm holding 

 this stop be carefully turned aside so as not to disturb the 

 focus in the very least, it will be found that they occupy 

 precisely the position of the focus for the ordinary white dots. 



We must all be in agreement with Mr. Stokes' conclusions, 

 but this new experiment shows that some other demonstration 

 is needed than that of a stop at the back of the condenser, for 

 the purpose of illustrating the effect of spherical aberration on 

 the image. The whole question is a most difficult one, and 

 fresh complications are always turning up, for in one of Mr. 

 Morland's slides of Pleurosigrtia cmgulaturii examples of that 

 diatom show both white above, and black below-, and others 

 black above, and white below, the diatoms facing the same 

 way up. 



In another slide of P. angulatwni mounted in dense medium, 

 the valves being all the same way up, show white above, and 

 black below, which is also the appearance when the valves 

 are mounted dry on cover. A narrow crack in a valve, mounted 

 dry on cover, is black when the dots are white, but in a medium 

 it is, as Mr. Stokes points out, white when the dots are white. 

 I have lately seen a balsam-mounted slide which has a 

 P. formosuni and P. rhomheimi both giving white above, and 

 black below, but on the P. rhomhewni there is a dark patch 

 in which this order is reversed. (These patches are by some 

 authorities called gum, and by others air.) It is important to 



