400 Annals New York Academy of Sciences 



chemicals used in cleaning and liberate these slowly into the water in which 

 they are washed or stored. If these chemicals are present, they cause the 

 diatoms to stick to the mica. It is, therefore, desirable to leave the diatoms in 

 at least the last two washing waters for a period of 2 days or more, and to pour 

 off the water in which they have been stored and replace with fresh distilled 

 water immediately before the preparation of the strews from which specimens 

 are to be selected. 



The actual selecting is most conveniently done under a binocular dissecting 

 microscope at a magnification of about XlOO. Except with the larger forms, 

 it is not possible at this magnification to recognize the species to be selected with 

 certainty. It is, therefore, necessary to locate them under an ordinary micro- 

 scope and to note their position relative to prominent specimens that can act 

 as markers. This process is facilitated if a grid is ruled on the back of the slip 

 of mica with the point of a needle and the scratches filled with India ink. The 

 micashp can then be mounted with balsam on a microscope slide. It is usually 

 more convenient to assemble specimens of each species to be investigated in 

 separate groups near the edge of the mica shp before transferring them to the 

 grids. When small diatoms are being dealt with, each group can then be 

 examined under the ordinary microscope to see that all the specimens are of the 

 correct species. 



The necessary number of formvar-coated grids are attached to an ordinary 

 microscope sUde by tiny drops of gum arable at their edge. They are held 

 steady by this during mounting but can be readily detached for insertion in the 

 microscope. A label can be placed at one end of the slide giving a numbered 

 key to the grids. The diatoms can then be taken up individually on a bristle 

 from the mica shp and placed on the formvar film over the spaces in the grid. 

 This can usually be done without tearing the film. When the work is done in a 

 dry atmosphere, the diatoms at times accjuire an electrostatic charge, which 

 causes them to fly off the grid when it is lifted off the slide. This trouble can 

 be obviated by breathing gently on the grids after the diatoms have been trans- 

 ferred to them. After the thin film of water thus condensed on them has 

 evaporated, they adhere sufficiently not to fall off when the grid is placed in 

 the electron microscope, and will normally remain in position through a number 

 of insertions into and removals from the instrument. 



I find it possible to transfer the diatoms freehand, even specimens the major 

 axis of which is between 10 and 15 n in length. For this I use a bristle mounted 

 on a cylindrical rod of wood about as thick as a pencil and sharpened like one 

 to a point at one end. The bristle is stuck to this point with about 2 mm. 

 protruding. Pelletan (1888) and Hustedt (1927) recommend a pig's eyelash 

 as the most suitable bristle and I find one very satisfactory. 



A number of types of mechanical fingers for the selection of diatoms have 

 been developed, the most widely used probably being that designed by Meakin 

 (1939). These could no doubt also be used for transferring diatoms to electron 

 microscope grids. Stiffer bristles than those used for freehand mounting are, 

 however, normally used in mechanical fingers and these would be more likely 

 to tear the formvar films on the grids. When a mechanical finger is used to 

 mount diatoms for the electron microscope it will probably be advantageous to 

 replace its normal bristle by a more flexible one. 



