MEMORANDA. 1 59 



parallel to, and coincident with the axis of the object-glass, the 

 cone of rays will be bisected, and the opposite sides of the 

 surface brought into the eye simultaneously : consequently, a 

 somewhat confused definition is the result. A protuberance 

 on one side will seem to penetrate and project through on the 

 other side. With the aid of a diagram it would be easy to 

 demonstrate mathematically the optical principle to which this 

 appearance is due, but in the present case, a familiar ex- 

 ample will, perhaps, suffice to prove the fact. 



In viewing the circulation of the Anacharis with a large 

 aperture, the chlorophyll granules traversing along a straight 

 and thin septum, (if the position is favourable,) appear to 

 project into the neighbouring cell, seeming to pass directly 

 under the line of the cell-wall. Smaller particles will 

 apparently travel within the substance of the wall, and in case 

 of a boundary or single cell, or in unicellular plants if the 

 surrounding water has nearly dried up, the rim or prism 

 remaining round the exterior, causes irregular refracted 

 images of the particles of protoplasm to appear outside the 

 cell, bearing such a remarkable similarity to external cilia, 

 that the passing shadows may even be mistaken for currents 

 in the water ; I do not say positively that these are the causes, 

 giving rise to the appearance of cilia observed by others, I 

 merely mention them as facts to be borne in mind. 



I may also state that I have never been able to discover the 

 orifice, said to have been seen at the extreme ends of the 

 Closteria. It may be assumed that if such an opening existed 

 it would have something like a structural margin, of such a 

 size as to allow its position at least to be visible under the 

 microscope, but not the slightest break can be observed in 

 the laminated structure that the thickened ends display. 



All attempts that I have made to ascertain the existence of 

 cilia in the Diatomacea have been equally unsuccessful. How 

 then are the active traversing motions of these organisms to 

 be accounted for ? If caused by the action of cilia, such 

 extreme! V rapid impulses would be required, to propel the 

 comparatively large body through the water, that surrounding 

 particles would be jerked away far and wide ; a similar effect 

 would be observed if the propulsion was caused by the reaction 

 of a jet of water ; which, according to known laws of hydro- 

 dynamics, must necessarily be ejected with a rapidity sufficient 

 to indicate the existence of the current, a long distance astern. 

 I consider that tliere is no ground for assuming the motions 

 of the Diatomacece to be due to either of these causes. They 

 are urged forward through a mass of sediment, without dis- 

 placing any other particles than those they immediately come 



