894 SUMMARY OF CURRENT RESEARCHES RELATING TO 



gutter very deep, but closed towards tlie interior (as other observers 

 admit *) by a fine membrane, the least violence will be sufiicient to 

 break the section at this point and give the image of a fissure travers- 

 ing the thickness of a valve from one part to another. This effect 

 cannot have been produced in the case of the section previously 

 described, because it is supported by the solid portions of the valve 

 farther off, and above all by the hardened balsam which surrounds it. 

 An absolutely solid cement does not present these inconveniences. 

 Balsam is only suitable for giving, to an already consistent rock, 

 greater hardness. It could not bo employed to agglutinate diatoms 

 in powder. The other cements which 1 have used have given me 

 almost negative results. There is, however, one the use of which 

 would, I think, give some advantages ; it is the solid matter deposited 

 by certain petrifying waters. The waters of these springs, which are 

 sometimes employed to obtain remarkably delicate copies of medals, 

 bas-reliefs, &c., leave, on evaporating, a hard translucent substance, 

 composed in great part of carbonate of lime. On mixing a certain 

 quantity of the frustules with these mineral waters, we should obtain, 

 by evaporation, a deposit in which very thin and perfectly transparent 

 laminae could be cut and sections of diatoms obtained. Moreover, the 

 cement could easily be removed by a weak acid, which would enable 

 the sections to be mounted, isolftted in a medium more favourable to 

 their study. 



It is not even necessary to have recourse to these artifices of pre- 

 paration. Many sufficiently hard rocks contain diatoms in more or 

 less considerable quantity. Certain varieties of guano, for example, 

 are very hard and give very good sections. Here is a still virgin 

 field which will furnish many an interesting observation to those who 

 will explore it." 



MICROSCOPY. 



o. Instruments, Accessories, &c. 



Beck's Pathological Microscope. — Messrs. Beck have designed 

 this Microscope with a special view to delicate pathological research. 

 The instrument is on the same model as their Economic Stand, but 

 to it has been added a rack-and-pinion substage with centering- 

 screws, which carries an achromatic condenser of an aperture of about 

 1'4 N.A. It is supplied with two rotating diaphragm-plates, the 

 upper containing a series of blue glasses for moderating the light, 

 the lower a series of openings of different sizes, by which the aper- 

 ture can be varied to any extent, which are also placed at a distance 

 balow the lenses sufficient for accurate centering of the condenser. 



Of this arrangement Messrs. Beck say, " This convenient method 

 for rapidly varying the intensity and angle of the cone of light by 

 means of the two diaphragm-plates will, we feel sure, be appreciated 

 by all practical workers on minute pathology. We have made the 

 lenses of large diameter, so that a great flood of light can be used when 



* Schmidt, Bot. Ztg., 1872, p. 741. 



