554 Notes. 



phenomena of reflection or of refraction on its surface) ; and it 

 reappears whenever the ratio of the two indices is greater or less 

 than unity. To demonstrate depolarisation, then, one must mount 

 the object in a series of liquids of gradually increasing refractive 

 index, and ascertain whether its illumination diminishes, reaches 

 a minimum, nil (when its index is equal to that of the liquid), 

 and then gradually increases again. 



We have continued, on various contractile organs, and by the 

 same immersion method, the comparison of the phenomena of 

 birefringence and depolarisation, as begun by Vies. Our observa- 

 tions were carried out on (1) various motor elements of Protozoa 

 (cilia and myonemes of ciliate Infusoria) ; (2) flagella of sperma- 

 tozoa; (3) swimming-plates of Ctenophora ; (4) the body, and that 

 much-discussed structure, the undulating membrane of Trypano- 

 soma balbianii Certes. 



The several experiments were made on preparations either 

 mounted separately in each of the liquids of the series and then 

 compared together, or else passed through all the series in succes- 

 sion, first in one direction and then in the reverse. The two 

 methods gave comparable results. 



Concerning the latter method, however, it is necessary to 

 point out here that if one follows one of these " reversible " prepara- 

 tions first in one direction and then in the other through the 

 ascending series of indices, the intensity of the illumination of the 

 same object in the same liquid is not always exactly of the same 

 degree on the outward as on the return journey ; there is a sort of 

 retardation, a "hysteresis" of the illumination, which, after all, 

 is exactly what one might expect, seeing that the process has to 

 be carried out under a cover-slip ; the diffusion of the new reagent 

 introduced, and the corresponding elimination of the old, is never 

 quite perfectly effected, and there may well be traces of the pre- 

 ceding liquid left (of higher or lower index as the case may be). 

 As a result, the numerical equivalent of the preparation-liquid is 

 in reality a little higher or a little lower than its true value, 

 according to the direction of the progression through the series. 

 Moreover, these differences become less and less with the length of 

 time that one allows for the diffusion of the liquids. 



A. Cilia of Pkotozoa. 



Our experiments were carried out on the adoral cilia of large 

 Stentors (Stentor polymorphus Ehrbg.) and Vorticella. The results 

 are entirely in agreement with those obtained for the cilia of 

 the gills of the mussel. The reaction is very near zero (so 

 near, indeed, that very delicate methods of compensation alone 

 are able to make it appreciable) in a zone of indices included 



