806 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



with adsorbed gas, and later during the deposition is freshly condensed 

 silica. Films are also found in shadows of replicas made of other materials 

 such as silicon monoxide, germanium, gold-manganin, and even chromium. 

 Whether the interpretation should always be the same as for silica, or 

 whether in some cases the diffusing component is different from the sticking 

 component, is uncertain. 



A high degree of contrast is commonly attributed to siUca replicas, which 

 are supposedly deposited at near-normal incidence. In the writer's experi- 

 ence, distortion of the conical-tungsten-basket siUca evaporator often re- 

 sults in values of Ba greater than 10°. If normal incidence, characterized by 

 absence of shadows, is actually attained, the resulting repHca exhibits 



Fig. 5 — Silica replica of natural surface of thermistor flake heated to 1425 °C. Note 

 difference in contrast between the two pictures of the stereogram. 



poor contrast unless steep slopes are present. An example is shown in Fig. 

 5, in which difference in contrast due to the fact that da differs for the two 

 pictures of the pair by approximately 8°, the stereoscopic angle, is evident. 

 Note that the shadow in the surface feature in the center of the grain at 

 the right is more pronounced in the left-hand picture which shows the 

 greater contrast. The replica is from the surface of a thermistor flake, 

 sintered briefly at 1425°C. 



Figure 6 is from a silica replica about 400 A thick, of a portion of the 

 surface of a thermistor disk sintered 6 hrs. at 1175°C. The white line is 

 0.1 /i long. The striations show a minimum separation of about 250 A, and 

 the character of the shading indicates that this is near the limit of resolu- 

 tion of the micrograph, about 200 A (Table I). Higher resolutions claimed^- ^ 

 are probably due either to shadow effects, to special situations on the sides 

 of steep slopes, or perhaps to the use of extremely thin replicas. 



