158 T. a. o'donohoe on the minute 



representations of the same structure. The next two slides show 

 the inner membrane projecting beyond the outer one, and on 

 examining the edge of the fracture it becomes at once evident 

 that what is called the eye-spot is a comparatively large 

 perforation. This membrane also shows considerable thickness. 

 I have been able to find a small fragment in which the outer 

 membrane projects a little beyond the inner layer. This is seen 

 by the fact that the silex of the projecting part appears white. 

 It should now be noted that this white silex is sharply defined at 

 the edge, that this edge shows hardly any thickness, and that the 

 perforations are represented by black dots. The next image has 

 been obtained by making no other alteration than that of raising 

 the objective until the white dots appeared. On examining this 

 image we find the white silex has become black, and the edge of 

 the fracture which was so well defined in the black-dot picture is 

 now so blurred and fogged as to have become invisible. We have 

 next two fragments of Pleurosigma angulatum in juxtaposition, 

 showing respectively the black and white dots. The black dot 

 image gives the "postage-stamp" fracture well defined in white 

 silex, whereas the broken edge in the other fragment is black, 

 out of focus and blurred. We are therefore justified, I think, 

 in relegating the white-dot images of diatom structure to the 

 abode of Mr. Nelson's ghosts. Here, methinks, I hear the tyro 

 in microscopy cry out, " If that be so, why do we meet with 

 so many white-dot images in the books which are written for 

 our guidance ? " I prefer to let the writers of these books 

 answer for themselves. Mr. Pringle, in Practical Photomicro- 

 graphy, 1890, page 173, writes: "In spite of all these details, 

 A. pellucida is child's play to photograph in comparison with 

 such tests as Pleurosigma angulatum, Surirella gemma and 

 Navicula rhomboides by axial light and to show ' black dots. 

 Pleurosigma angulatum in white areoles, or Navicula rhomboides 

 in squares, with a special disc in the condenser, is infinitely 

 easier than the same in black dots." 



Let us turn now to Plates III. and IV. of Dr. Spitta's Photo- 

 micrography (1899). What do we find? White-dot images of 

 all his diatomic tests. Not one black-dot image ! He has, no 

 doubt, some good reason for this, and turning to page 138 we 

 find it. Writing about the photography of Pleurosigma 

 angulatum, Dr. Spitta says : " It has two principal planes of 

 focus, and much difference of opinion exists as to which is the 

 correct one. The last picture taken by Dr. Van Heurck with 



