ELKCIKON MKHOSCOrV 



Fig. 7. Needle of silver azide undergoing ther 

 mal decomposition. (X 10,000 horizontal, © = 25°). 



Fk;. 8. Surface structure of newsprint. (X600 

 horizontal, © = 45°). 



the performance of dynamic experiments 

 such as studies at elevated temperatures. 

 One example is shown in Figure 7. In this 



study (22), needles of silver azide were 

 grown on a silver disk which could be heated 

 in tlu; microscope. The micrograph shows 

 that thermal decomposition is proceeding 

 from the end of the needle making better 

 contact with the disk, leaving behind a 

 pebble-like structure. 



The conventional electron microscope may 

 also be adapted for the direct examination of 

 surfaces of specimens by the well-known 

 reflection method. A comparison of this 

 method with the scanning technique shows 

 that the obtainable resolution is about the 

 same but that the latter possesses certain 

 advantages. First, while the angle of observa- 

 tion used in the reflection method is usually 

 about 10° with a maximum of 25° using a 

 refined technique and in special circum- 

 stances (23, 24), in the scanning method an 

 angle of about 45° is used, hence foreshort- 

 ening and obscurity in the image is much 

 reduced. Second, contrast formation in the 

 scanning instrument is not primarily a proc- 

 ess of shadow formation. Unlike the re- 

 flection method, therefore, in which the angle 

 of illumination is only a few degrees, no 

 dense foreground shadows are formed to 

 obscure parts of the surface. Third, the 

 beam intensity in the scanning instrument 

 is extremely low and quite delicate speci- 

 mens of fibers and other material may be 

 examined without damage. The silver azide 

 specimen, for example, would be decomposed 

 instantly under the high beam intensity of 

 the conventional instrument. Figure 8, 

 showing the surface of newsprint paper, is an 

 example of a very rough and, at the same 

 time, delicate type of specimen. 



Quantitative information concerning the 

 micro-geometry of a surface may be obtained 

 by taking stereomicrographs and by taking 

 micrographs at two different observation 

 angles. The specimen stage of the scanning 

 instrument is, therefore, provided with tilt 

 and rotation as well as the usual traverse 

 motions. 



Another special application of the scan- 



250 



