256 E. M. NELSON ON THE INTERPRETATION Otf 



others that it resembles a string of sausages with constrictions 

 in it. This difference in the interpretation of the images of 

 the same object is mainly to be accounted for by variations in 

 focus and adjustment. If such differences are present in these 

 comparatively easy objects, we may expect to find them greatly 

 increased when vieA\ ing objects that present greater difficulties 

 in the determination of focus and adjustment. 



Sucli are diatoms, owing probably to the transparency of the 

 silex of which they are composed. To find the truth of this 

 one has only to turn to the extensive literature on what may be 

 termed " the resolution of the markings on the diatomacea?," 

 and he will find such expressions as striae, checks, areola?, 

 puncta?, white dots, black dots, hemispherules, pearls, beads, 

 etc., used to denote the same thing. Now let us take the 

 Pleurosigma formosum, and, setting aside the stria? and 

 diamond-shaped markings as quite exploded things of the 

 past, apply ourselves to the consideration of the more recent 

 interpretations. There are two kinds of markings which we 

 shall have to discuss thoroughly. I name these the white dot, 

 and the black dot. The white dot is usually accepted as the 

 critical image of this, and other diatoms. 



To show that this is so, let me refer you to the photograph 

 of Navicula rJiomboides by Dr. Woodward, copied in the 

 "Monthly Microscopical Journal' for May, 1876; also to his 

 photograph of Pleurosigma angulatum, copied in Dr. Van 

 Heurck's " Synopsis." Both these show the pearls, beads, 

 hemispherules, or what I have called the white dots. I 

 endeavoured to show that these appearances were erroneous, at 

 the demonstration I gave here on March 14th, 1884, but could 

 not, for want of time, go into the matter at sufficient length ; 

 I hope you will pardon me therefore, for again referring to it. 

 When examining a P. formosum, in balsam, with the black dot 

 resolution, I noticed that one or two of the dots were very pale. 

 It appeared as if those identical dots, or hemispherules had been 

 removed, leaving the plain silex underneath. On changing the 

 resolution for that of the white dot I found all the dots equally 

 perfect. I could not tell where the damaged beads were 

 situated. There is not a shadow of doubt in my mind as to 

 which of the two is the truer picture. The black dot which 

 differentiates between one or two of the markings and the rest 



