ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 651 



diatoms in the gathering, but saw nothing of the sort except in the 

 instances described. The Naviculce were very lively, but he saw no 

 examples of action upon foreign matter that came in their way. 

 Neither could he detect any current, even along the Nitzschias ; the 

 motion of the gelatinous substance occurring only when it came in 

 contact with the shell and apparently adhering to it. 



The study of the diatom-shell has led the author to accept the 

 opinion that the raphe is a real fissure in the shell, but in many 

 species it is not a simple and vertical linear-opening of the shell. It 

 is more like the joint formed by the overlapping of the edges of 

 curved tiling on a roof : a thickened line of silica borders one lateral 

 half of the shell, while the other half dips under it with a thin film. 

 It is true that an osmotic force may be conceived as working along 

 the raphe, as well as that a line of cilia should do so ; but the 

 difficulty is to account for such action upon an extraneous mass as that 

 described, or to make osmosis from such a place upon the shell move 

 the diatom in the direction of its length. The assumed presence or 

 absence of a gelatinous film enveloping the diatom does not mate- 

 rially vary the conditions of the problem in either case. If we 

 assume that the osmotic action is at the extremities of the shell, the 

 observed phenomena, as to the action upon the gelatinous mass when 

 in the middle of the frustule, are unaccounted for. 



As to the manner in which the lapping of the halves of the 

 frustule along the raphe is effected, it may be most easily seen in 

 some of the coarser Pleurosigma. In broken shells of P. attenuatum 

 and P. formosum, it was seen very plainly demonstrated. Sometimes 

 the thickened line of silex which borders one-half of the frustule 

 will be found sticking out alone, the thinner part of the shell being 

 broken away from it. Sometimes it will be in its normal position, but 

 the lateral halves of the shell will be separated by pressure, so as 

 to show on one side the thick edge, and on the other the fitting 

 gutter caused by the projection of a thin lip. Occasionally also a 

 cross fracture of the shell will be found on a broken fragment, in 

 such position that we get the benefit of a cross section, and see the 

 whole joint in the form described. 



MICEOSCOPY. 

 a. Instruments, Accessories, &c. 



Ahrens's Erecting BinocTilar Microscope.— Fig. 135 shows a form 

 of erecting binocular devised by Mr. C. D. Ahrens. The Microscope 

 as figured is intended only for use with low powers, as there is no 

 fine adjustment. 



A sliding-box c contains two WoUaston prisms (such as are used 

 for the camera lucida) ; & 6 are similar boxes containing each a 

 right-angled prism ; a a each contain a truncated equilateral prism 

 that directs the image pencils up the respective inclined tubes ; d d 



