SCANNING 



from 15 sec to 6 min has been found con- 

 venient; the shorter recording times are 

 used for low magnification work when high 

 spot-currents may be emploj^ed. 



\\Tien it is desired to examine a particular 

 portion of a chosen field at a better signal- 

 to-noise ratio but at the same magnification, 

 facilities are provided for reducing the size 

 of the picture and the area scanned in the 

 same ratio and for shifting the raster to ex- 

 plore different regions of the original field. 

 This feature is also useful for critical focus- 

 ing. 



Because of the D. C. shift and low record- 

 ing speeds involved, the time bases are direct 

 coupled throughout. 



The resolution of the long persistence 

 screen is not high, and it has been found to 

 be more satisfactory to record the picture 

 using a second tube possessing a short per- 

 sistence high-resolution screen. This tube is 

 conveniently photographed using an oscillo- 

 scope-type camera and 35-mm film. An 

 additional advantage of the two-tube display 

 is that during recording the picture may be 

 monitored on the direct display tube. 



Applications of the Scanning Micro- 

 scope (21). The examination of the surfaces 

 of small specimens and the interpretation of 

 micrographs are rapid and simple in the scan- 

 ning microscope as compared with the more 

 usual replica method used with the trans- 

 mission microscope, although these facilities 

 are gained at the expense of a somewhat 

 inferior resolution. No preparative tech- 

 niques are involved for metal specimens 

 but non-conducting specimens have to be 

 metalized by means of the vacuum evapora- 

 tion of a 100-500 A thick layer of gold-palla- 

 dium. A method of examining non-conduct- 

 ing specimens in the scanning microscope, 

 without metalizing, has been demonstrated 

 (27) in which the beam accelerating \'oltage 

 is reduced to the order of 0.5 — 2kV. In this 

 voltage range the secondary emission ratio 

 stabilizes at unity and the surface of the 

 specimen remains at ground potential. The 



Fig. 6. Worn facet on one knuckle of a woven 

 phosphor-bronze wire mesh (Fourdrinier ^-ire) 

 used in the paper-making industrj'. (X200 hori- 

 zontal. O = 45°). 



reduction of resolving power at lower ac- 

 celerating voltages is partially offset by the 

 higher secondary emission ratio. 



The scanning method may also be applied 

 in the case of very rough surfaces containing 

 re-entrant features, surfaces of a loose par- 

 ticulate nature and for specimens of intricate 

 shape; in all these cases the replica method 

 may be difficult or impossible to apply. The 

 scanning instrvmient may be operated at 

 magnifications as low as X200 an in this 

 range may be advantageous as compared 

 with the light microscope because of the 

 great depth of field obtainable. 



Some of these advantages are illustrated 

 in Figure 6, which shows a worn facet of a 

 Fourdrinier wire mesh upon which newsprint 

 is formed. Scale-like corrosion products can 

 also be seen in this micrograph. 



Being a direct method of examination, the 

 scarming instrument is particularly suited to 



249 



