330 N. E. BROWN ON THE STRUCTURE OF DIATOMS. 



ance I have tried to represent at the lower part of fig. 10. I 

 believe that each of these dots, or minute meshes as they really 

 are, is closed by an extremely thin membrane of silex, as on one 

 occasion, when using a dim light reflected from the cover-glass 

 upon the diatom, the presence of such a membrane seemed to be 

 very distinctly evident by the light reflected from its surface over 

 each dark spot and nowhere else. But I entirely failed to see 

 the slightest trace of a pore in it, although I think it probable 

 that one exists in each mesh. 



Nitzschia scalaris. When the fine striae on this diatom are 

 magnified up to 3,000 diameters, they are seen to consist of 

 small beads or pearl-like dots of silex, which are either black or 

 white according to illumination. Upon the very thin membrane 

 between each pair of these rows of beads, a row of very minute 

 pores is just discernible, as represented at fig. 11, which is 

 drawn with a camera-lucida, using central light and a green 

 screen. Under the best of circumstances they are exceedingly 

 faint, and I am not at all sure that they are accurately spaced 

 in my drawing, as I found it exceedingly difficult to plot them on 

 paper by means of a camera-lucida ; but the drawing is suffi- 

 ciently accurate to show their position. It requires good eyesight 

 to see them at all, and I do not think they would be visible at a 

 less magnification than 2,500 diameters. The light must be 

 most carefully manipulated, and for my vision I have found 

 them to be most evident in a rather dim light, a glare effaces 

 them ; also at a very slight touch of the fine adjustment they 

 instantly vanish. As a test for high powers, manipulative skill 

 and keenness of vision, I think few things can be found more 

 suitable than the resolution of the pores of this diatom when 

 mounted in styrax. 



Amphipleura Lindheimeri. When the surface of this 

 diatom is accurately in focus (not the black-dot view), a fine 

 grating with square meshes is seen, which somewhat resembles 

 that of Surirella gemma ; the bars transverse to the raphe being 

 straight, whilst those parallel to the raphe form sinuous lines, 

 because the ends of the short partitions which divide the space 

 between each pair of transverse bars into square meshes do not 

 exactly coincide with the ends of the partitions between the 

 adjoining pairs of transverse bars. At a magnification of 3,000 

 diameters, when the membrane covering the meshes of the 



