284 



:jr— "s^T ■ ^^L-J, 





Fig. I. Cellulose acetate fibre at the place where it was cut 

 with scissors. Maenitication 3000. 



Fig. 2. Diatomaceous powder sprinkled on to swollen gelatin. 

 Magnification 3000. 



fibres with random orientation will be described. 

 The method has been designed so that no skilful 

 or patient manipulation of individual fibres is re- 

 quired. 



Short pieces of the fibre are dropped on to the 

 swollen gelatin of a fixed and washed photographic 

 plate. This may be done most conveniently by 

 cutting lengths from the fibre or yarn while this is 

 held above the photographic plate and allowing the 

 pieces to fall on the gelatin. Surface tension will 

 cause the swollen gelatin to creep up the sides of the 

 fibres. After the gelatin has dried the fibres will be 

 partially embedded in it. 



Marco Resin' to which a catalyst and an accele- 

 rator have been added, according to the makers" 

 formula for cold curing, is spread on a microscope 

 slide, which is then laid on the gelatin surface con- 

 taining the fibres, so as to form a sandwich: glass- 

 gelatin-fibres-Marco Resin-glass. This is then left 

 until the resin sets. At room temperature the setting 

 time varies from 40 minutes to 3 hours depending 

 on the accelerator content. Roughening one side of 

 the microscope slide by grinding promotes the adhe- 

 sion of the resin to the glass. A photographic plate 

 slightly larger than the microscope slide is used and 

 the slide is placed so that part of it overlaps the edge 

 of the photographic plate. 



By pressing the photographic plate with thumbs 

 and the projecting end of the slide with forefingers 

 the assembly is mechanically separated along the 

 interface gelatin-Marco Resin. A similar method 

 of separating Marco Resin from gelatin has been 

 used by Dew (1) in the production of optical replicas 

 of diffraction gratings. It is now found that all 



fibres have been transferred to the resin. The flat 

 surface of the Marco Resin — the cast of the photo- 

 graphic gelatin — from which in numerous places 

 fibres emerge is now replicated using a modification 

 of the technique described earlier (2). 



A layer of silver about 0.1 // thick is deposited by 

 vacuum evaporation on the resin matrix and is 

 subsequently freed by immersion of the whole for 

 a few minutes in chloroform. This intermediate 

 replica is then coated with the final replicating mate- 

 rial again by vacuum evaporation. 



The final replica may be of silicon monoxide or 

 carbon but a better contrast in the image is obtained 

 if the replica is made of gold-palladium alloy or one 

 of the other metals used for shadow-casting which 

 will not be attacked by nitric acid. 



To prevent the replica film from being damaged 

 during the succeeding operations it is backed, while 

 still on silver, with a thin film of nitrocellulose. The 

 silver is then dissolved in nitric acid and the backed 

 replica film, after washing in distilled water, is 

 eventually collected on specimen grids in the usual 

 way. Finally the backing film is removed by immer- 

 sion of the specimen grid in acetone for a minute or 

 so. 



References 



1 . Dew, G. D., The Nat. Phys. Lab., private communication. 



2. Ramanathan, N., Sikorski, J., and Woods, H. J., 



Biochim. Biophys. Acta 18, 323 (1955). 



1 Marco Resin SB.28C obtainable from Scott Bader & Co. 

 Ltd. 



