INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



1033 



diaphragm, and having its lower aperture turned in the direction of 

 the currents. The apparatus set upon a vertical axis is acted upon 

 by the feeblest currents. A Robinson anemometer serves to measure 

 the velocity of the air during the experiment, and to estimate 

 approximately the volume which has passed through the apparatus. 



Both forms were originally constructed of glass, but are now 

 made of copper nickelled. 



Microscopical Appearance of the Valves of Diatoms.* — This 

 paper by Mr. Julien Deby is written with the view of enabling micro- 

 scopists more readily to interpret the appearances presented by diatoms 

 as seen under the Microscope, which is often very difficult with trans- 

 mitted light. By an attentive examination, however, of the details, 

 " even the most incomprehensible problems will be resolved as if by 

 enchantment." 



Taking Niti^chia as the first type, the author divides the different 

 forms into two principal divisions — those in which the two sections of 

 a valve meet at an acute angle, and those which meet at an obtuse 

 angle, each of which divisions may have the valve-sections plane or ' 

 curved. Tliese four divisions may each be subdivided into three others, 

 according as the midrib is central, normally excentric, or submarginal 

 (that is, with one of the valve-sections nearly obsolete). 



The author then gives diagrams of the appearances of various 

 forms, of wliich we can only subjoin one — Fig. 107 [sm., im., upper 

 and lower midrib; o, connective). This 

 shows a Nitschia with valves whose two 

 sections form an acute angle, and witli 

 an excentric midrib. Taking the line 

 which represents the surface of either 

 of the larger sections as a horizontal, 

 and drawing tlie perpendiculars, we 

 shall have the true microscopical pro- 

 jection of the diatom when viewed from 

 the upper or under side respectively. 



The author considers tliat it is 

 necessary to pay more attention to 

 details tlian has liitherto been the case 

 in this difBcult genus, and in defining 

 every species (1) to describe the general 

 form of the frnstule, and tlie relations 

 of the length of the two sections of tlie 

 valves to the breadth ; (2) to indicate 

 the position of the midrib by reference 

 to the imaginary central line of tin 

 valve, that is to indicate the relative 

 size of the two sections ; (3) to deter- 

 mine the number of stria? by reference to tlic nodult-s of tlio midrib; 

 (1) to count the siliceous grains of the midrib, the striie of tlio valve 

 sections, and the number of points per stria ]>er micro-millinietro ; (.'")) to 

 indicate if the valve is acute or obtuse, and its sections plane or curved. 

 * Sep. rcpr. ' Ann. Soc. n,!;;. Micr.,' v. (1880), Mt-ni., IH pp. aii.l 20 figs. 



Fio. 107. 



