ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 413 



is only affected by the direction of the motion of the gelatine on one 

 valve — the one that comes in contact with a surface. The motion is 

 only on the sides, or valves. If a frustule is turned with its so-called 

 front view up, it cannot move, at least those under my investigation 

 did not, unless one of the valves comes in contact with some body of 

 greater weight than itself, if we may so put it ; for small particles 

 will move along the valves, while the diatom remains stationary. The 

 ribs have much to do with this power to creep, for the pallium is 

 folded to the ribs, and being striated lengthwise it is plain to see how 

 the pallium covers the valves with thousands of little feet. There is 

 great room here for investigation with the recent very wide-angled 

 glasses, but let me here give common glasses their due, for all that I 

 have discovered has been done with only such." 



Pfitzer's Diatomaceae.* — In his account of the Diatomaceee con- 

 tributed to Schenk's ' Encyklopaedie der Naturwissenschaften,' E. 

 Pfitzer commences with the simplest types, such as Pinnularia, pro- 

 ceeding then to the complicated structure of some marine algfe. With 

 regard to their power of spontaneous motion, he adopts Schulze's view 

 that it is due to extensions of the protoplasm which protrude through 

 fissures in the cell-wall, although these have not yet been actually 

 detected ; rejecting that of Mereschkowski, that it is the result of an 

 osmotic process. The jerks and vibrations described by this latter 

 writer as observable in bacteria which are located close to the suture 

 of diatoms, Pfitzer states are in no way due to motile processes. The 

 ordinary mode of fission is then described, as well as the formation of 

 auxospores. As regards their systematic position, Pfitzer regards the 

 diatoms as most nearly allied to the Schizophytae rather than to the 

 Gonjugatae. 



MICEOSCOPY. 

 a. Instruments, Accessories, &c. 



Bertrand's Petrological Microscope.f— This instrument (fig. 60) 

 is designed by M. Bertrand, the Director of the ' Comptoir Mineralo- 

 gique,' at Paris, and has several specialities : — 



Above the objective at F is inserted a slide L with an achromatic 

 lens so as to use either parallel or convergent light as may be desired, 

 the slide being raised or lowered by a rack-and-pinion movement, the 

 milled head of which is shown at P. A slow motion is given to the 

 rotating plate of the stage by a tangent screw terminating in the 



* Pfitzer, E., 'Die Bacillaiiaceen ' (Diatomacese). 



t Trutat's ' Traite' elementaire du Microscope,' 1883, pp. 266-70 and 300-1 

 (3 figs.). In addition to the references given in the text, A is the eye-piece, R 

 the milled head of the coarse adjustment, w' the centering screws for the 

 objective, V V the milled heads of the stage movements, and B that of the sub- 

 stage tube, E, for the polarizer. At F can be inserted a mica qu;irtz-plate or a 

 quartz prism. 



