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



shade or half a shade, the brightness of the perforations varies 

 from many shades brighter than the field to nearly blackness. 

 When making these experiments it is of the utmost importance 

 that the silex be contrasted with the field, and not with the 

 perforations; for the silex looks ever so much brighter than 

 the black dots, and ever so much darker than the white dots, 

 but the change in the brightness of the silex when contrasted 

 with the field, which under such slight focal alterations as we 

 are speaking about undergoes practically no alteration, is quite 

 small. 



Now, if while the objective and illumination remain the same, 

 we change the diatom for one more finely marked, we shall 

 find that at the lower focus the perforations are not so black ; 

 and if a still finer one be examined it may not be possible to 

 see the black dot appearance at all, but only the white. So, 





>m o 



^ /^/.'^ 



'.■' -if/fjy y^ja^yjk^ 



c 







-f 



then, it is obvious that, for some reason or other, this white and 

 black dot appearance depends upon a ratio of the aperture of 

 the objective to the fineness or minuteness of the perforation. 

 Again, if we select a diatom with bolder perforations than the 

 N. lyra — say, for example, our old friend Actinocyclus Ralfsii — 

 we shall find that with the same objective and illumination 

 the perforations have a bright dot (Fig. a) in the centre 

 of the black dot ; this will be all the more marked if we use 

 an oil immersion of 1'43 N.A., and the full aperture of a 

 dry condenser. At the upper or white-dot focus we have 

 the complementary picture of a black dot in the centre of a 

 white one (b). This bright dot (a) is the image of the back 

 lens of the substage condenser, and on focussing up the condenser, 

 so that the image of the edge of the flame is out of focus in 

 the field, we shall find the image of the edge of the flame 



