436 SUMMARY OF CURRENT RESEARCHES RELATING TO 



cell of each end of the filament. In those species which have no 

 such sheath, variations in the turgidity are also brought about by 

 variations in the exosmotic and endosmotic phenomena of the cells. 



Alveoli of Diatoms.* — A. Grunow considers that the perforation 

 of the alveoli has been completely proved in diatoms from the 

 Jutland cement-stone and from the London clay ; but that this 

 can only modify the previously adopted interpretation, since he 

 considers that the diatoms from these localities have already begun 

 to undergo dissolution — this being unquestionably the case in those 

 from the London clay — in consequence of which the delicate closing 

 membranes of the alveoli disappear first of all. He believes that 

 whenever diatoms are accompanied by lime, and especially when 

 iron pyrites is present, as is the case in these localities, alkaline 

 reactions have been set up, which may have been very weak, 

 but which always act more strongly on silica than even very strong 

 acids. In valves treated with very strong acids the alveoli may some- 

 times be seen actually perforated ; while others close by will still 

 be closed. Coscinodiscus oculus-iridis and G. AsterompJialus — the former 

 of which (in cement-stone) has open, and the latter unquestionably 

 closed membranes — are so closely related to one another that no 

 sharp line of demarcation can be drawn between them. The author 

 has given a close examination to this group of diatoms from Franz- 

 Josef Land ; and believes that the alveoli are closed above and 

 below by delicate membranes proceeding from the thickening-ring. 

 In Triceratium Favus the upper one is sometimes furnished with small 

 spines. He considers it very improbable that in nearly related forms 

 there should be so great a diversity of structure as that between 

 perforation and complete continuity of the valves. 



MICEOSCOPY. 



a. Instruments, Accessories, &c. 



Hensoldt's and Schmidt's Simplified Readin|^ Mieroscopes.f — 

 Dr. C. Bohn refers to the necessity for insuring that the divisions of 

 the micrometer of these instruments t shall be a simple fraction of the 

 magnified image of the circle divisions. If the distance of the latter 

 from the objective is altered by moving the objective or the Micro- 

 scope, the power is no doubt changed, but the image no longer coin- 

 cides with the micrometer scale. Shifting the scale alone is of no 

 use, for the same reason. The only course is to alter the distance of 

 the objective from the circle divisions and the distance of the micro- 

 meter scale simultaneously in a proper ratio, the conditions for which 

 he discusses. A Eamsden eye-piece must be used. 



* Bot. Oentralbl., xvii. (1884) p. 67. 



t Zeitschr. f. Instrumentenkunde, iii. (1884) pp. 87-8. 



X See this Journal, ii. (1882) p. 548. 



