155 SOCIÉTÉ BELGE [IE MICHOSCOPIE. 



d'appui on the slide and coverglass. Any and every 

 inovement however, must be perfbrmcd at some disad- 

 vantage in so eonfined a space as that afïbrded by a very 

 shallow cell : a succession of normal movements is 

 scarcely possible under the circumstances. 



Another disturbing élément of even more importance 

 cornes into play during the microscopical investigation 

 of minute organisais. The siliceous skeleton of Diatoms 

 belonging to the filamentous order, although enve- 

 loped in a nearly structureless protoplasm, is found to 

 greatly interfère vvith the interprétation of structure, 

 while it increases the difficulty of arriving at the précise 

 nature and distribution of organs of préhension and 

 progression. Diffraction phenomena, it shouldberemem- 

 bered, are not eonfined to lined objects, nor to opaque, 

 semi-opaque or transparent éléments ; in short they are 

 universal whenever the strictly uniform propagation of 

 luminous waves is disturbed by the interposition of any 

 élément of unequal refraction. Diffraction phenomena 

 bave alwavs been a créât stumbling block in the way of 

 the interprétation of structure of every kind, especially 

 of the siliceous valves of Diatoms. In the présent case, 

 however, it appears to me to be simply necessary to limit 

 the action of the préhensile filaments to the boundary of 

 the plastic envelope, or a very little beyond it, and the 

 difficulty in the way of comprehending the movements 

 of the Diatomaeea 1 disappears, although they cannot 

 always be satisfactorily demonstiated. 



It bas been contendcd that the movements of Diatoms 

 areeither produced bycilia numeiïcally arranged through- 

 ou t the ventral longitudinal Une of the valve or pro- 

 jeeting from the « foramina » at theextremity. For some 



