150 PALMER : 



tration of his foam-structure visible in the living plasma.* 

 It is difficult enough, with a good i.g m.m. objective of 1.3 

 N. A., and with the necessary and correct achromatic illumi- 

 nation, what with all the corrugations of the shining silica 

 scattering rays of light, and the continued motion of the 

 diatom into ever- varying levels, to see even so much as I have 

 sought to depict in Figure II. These strands seem more or 

 less fibrous indeed, as if under some stress, and are of a not- 

 ably granular structure. They often appear to stretch slightly 

 into the keel, though it is impossible to define their limits in 

 the living condition. However, the chromatophor extensions, 

 instead of confining themselves to the lower half of the ribs, 

 frequently extend to the keel. Now it is a well-understood 

 fact that the chromatophors lie within the protoplasm, which 

 surrounds them on all sides. When, therefore, these tongue- 

 like projections reach the keel, the protoplasm must neces- 

 sarily extend still farther in the same direction. 



It causes me no pangs to confess my inability to see this 

 living protoplasm there, for the inference is plain that proto- 

 plasm may exist invisibly in the keel, whether because it 

 becomes hyaline in that position, like a fresh pseudopodium 

 of a rhizopod, or because the curved silicious tube of the keel 

 sadly interferes with the microscopical definition. 



However this may be, all signs of structure or motion 

 within the keel were long and most carefully sought in vain. 



*I trust I shall not be understood us entering here — Gott bew.ihre ! — 

 into a discussion of the Biitschli foam-structure of protoplasm. Hut cer- 

 tainly, in this case cited, I could not see the structure with adequate and 

 correct illumination. On abandoning the achromatic condenser (so 

 necessary in the opinion of the best informed experts of the microscope 

 for the formation of true images with high powers and apertures) and 

 using bright diffused daylight with the mirror only, I could indeed see 

 certain strong lines remotely resembling the reticulations in Figure I. 

 Curiously enough, they were nearly in the indicated place. However, 

 since these lines were demonstrably diffraction images, and since under 

 the conditions all real microscopical definition of the protoplasm structure 

 in the rib-strands was blotted out, I did not find the arrangement useful. 



