226 FRIEDLANDER AND PASCERI water to transfer the film to the grids, The carbon was caught on the grids as they passed through the water surface, Grid Shadowing with Platinum After being rotated in room air, the grid was carefully removed from the disk with sharp tweezers and placed ina petri dish to be shadowed with platinum. A blank grid, which was prepared along with the sample grid, was also set aside to be shadowed. The blank served as a check on the particulate contamination level in the substrate application process. The shadowing served two purposes: (1) It assured good visibility of particles not opaque to the electron beam, and (2) it provided a permanent impression of particles that might have been destroyed in the electron beam. The shadowing technique was similar to that described by Hall.’ The electron-microscope grids were placed in an evaporator pumped down to 0.054 Hg in about 30 min. Platinum evaporated from a wire wound about a tungsten support wire condensed on the grids. The effect of the size distribution of the deposited particles is not known. It would be of interest to vary the time of exposure of the grid to vacuum to see the effect on the measured distribution. Electron-microscope Examination Sample grids were examined with an RCA type EMU-3 electron microscope at various magnifications (18,200X to 5610X), depending on the particle size range under study andthe abundance of these parti- cles on the grid. Since the instrument had grid-position indicators, it was possible at each magnification to traverse a grid ina regular fashion, The magnification was determined before and after each set of traverses by photographing areas of a shadow-cast carbon-grating replica (28,000 lines per inch) with the magnification of the set taken as the mean of the two determinations. No micrographs were made of a region on which the substrate was broken, The photographic plates exposed in the electron microscope were examined with a 10-power Bausch and Lombeyepiece fitted with 0.1mm grating. The apparent particle diameter measured in this way included a contribution from the thickness of the platinum cap. Based on careful experimental measurements with shadowed polystyrene spheres, a correction factor of 40 A was applied to the apparent diameter. RESULTS The flux, dJ, of particles in the range r, dr to the grid surface is dJ = 0.62 D*vy~sn(r) dr =kn(r) dr (4a) (4b)