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Note on the Black and White Dot. 



By Edward M. Nelson, F.R.M.8. 



{Read Decemher 20th, 1901.) 



Two valuable papers upon this subject have appeared in the 

 last number of our Journal ; in presenting a third one for your 

 kind consideration I must, by way of apology, briefly explain 

 that, however puerile and insignificant this subject as indicated 

 by its title may seem, yet nevertheless any theory of microscopic 

 vision, how^ever well enunciated and proved, must be held to be 

 incomplete unless it yields an explanation of these phenomena. 



The description of the black and white dot which Mr. Stokes 

 in his excellent article has only given in general terms, might 

 lead those who are not perfectly familiar Avith the phenomena 

 to form wrong conclusions, unless it is clearly pointed out what 

 meaning the words are intended to convey. The statement that 

 at the upper focus the silex is dark and the perforations bright, 

 and at the lower focus the silex is bright and the perforations 

 dark, might give one the idea that the image was like a 

 tessellated pavement consisting of black and white tiles, where 

 a change from the upper to the lower focus would be represented 

 by taking up the black tiles and relaying with white, and the 

 white ones with black. This, however, would be an entirely 

 wrong conception of the images. Let us see exactly what the 

 appearances are like by an examination of a coarse diatom, say a 

 Navicula lyra, in balsam under a dry |^-in. objective with a | cone 

 illumination. At the upper focus we shall find the " lyra," 

 or clear part of the diatom, has a brightness just about equal 

 to that of the field outside the diatom, while the 2:)erf orations 

 are many shades brighter than the field ; then, at the lower 

 focus, we have the " lyra " portion perhaps one shade or half 

 a shade brighter than the field, while the perforations are 

 only a shade or two removed from blackness. So that while 

 the alteration in the brightness of the silex amounts to a single 



