SCANNING 



the tearing of the repHcas during the diki- 

 tion. When the pH of the solution in the top 

 of the funnel was approximately 5, the rep- 

 lica was placed on a grid. 



The diluting process could be accelerated 

 by removing the plastic block containing the 

 sample from the etchant after the replica 

 was released. The danger of the resulting 

 turbulence tearing the replica usually pre- 

 cluded the use of this time-saving step. 



This technique should be applicable to the 

 removal of carbon replicas from any solid 

 material which can be etched from beneath 

 the replica by acidic solutions, particularly 

 those containing hydrofluoric acid. 



V. J. Texnery, C. G. Bergeron, 



AND R. BORASKY 



RESINOGRAPHY. See p. 525. 

 SCANNING 



In the scanning electron microscope, an 

 electron optical system is used to produce a 

 fine electron spot which, in turn, is caused to 

 move over each point of the specimen. The 

 electron current leaving the specimen is 

 collected and amplified, and the resulting 

 signal is used to modulate a recording device 

 moving in synchronism with the electron 

 spot but with a greater amplitude. If the 

 specimen possesses some property which 

 causes the electron current leaving it to 

 vary from point to point, then a record will 

 be built up which will represent in some way 

 the variation of that property over the area 

 of the specimen which is scanned. The re- 

 solving power of such a microscope is de- 

 termined by the size of the electron spot. 



A microscope intended for the study of 

 secondary emission phenomena and based 

 upon the above principles was first proposed 

 by Ivnoll (1) in 1935. The first practical 

 scanning microscope was described by von 

 Ardenne (2, 3) in 1938, the main purpose of 

 this instrument being the examination of 



thick specimens in the transmission mode*. 

 This instrument, which used electrostatic 

 lenses and electromagnetic scanning, pos- 

 sessed no amplifying device; instead, the 

 electron beam, after passing through the 

 specimen, was allowed to fall on a film mov- 

 ing in synchronism with the beam but with 

 a greater velocity. The magnification was 

 given in this case by the velocity ratio of 

 the beam and the moving film. This early 

 instrument possessed several practical dis- 

 advantages among which w^ere very low effi- 

 ciency, necessitating recording times of 30 

 minutes or more, and no direct means of 

 observing and focusing the picture before 

 recording. 



von Ardenne suggested that living biologi- 

 cal specimens might be examined in air 

 by first passing the beam through a thin 

 Lenard window, and further that the sur- 

 faces of opaque specimens could be exam- 

 ined by collection and amplification of the 

 secondary electrons to modulate a cathode- 

 ray tube display. 



A scanning microscope designed specifi- 

 cally for the direct examination of the sur- 

 faces of metallurgical specimens was de- 

 scribed by Zworykin et at. (4, 5) in 1942; 

 it used electrostatic lenses and electro- 

 magnetic scanning of the beam. The speci- 

 men was placed with its surface in a plane 

 perpendicular to the axis of the objective 

 lens, and secondary electrons emitted from 

 the surface were accelerated back through 

 this lens to be detected by a fluorescent 

 screen followed by a photomultiplier. After 

 further amplification, the signal was applied 



* In thescanning instrument the electron beam 

 does not permit any focusing after its interaction 

 with the specimen; hence, energy losses occurring 

 in a thick specimen observed in transmission are 

 less serious than in the case of the conventional 

 transmission instrument. In practice, however, 

 the scanning method is only advantageous for one 

 particular type of specimen typified by highly 

 scattering particles lying on or near the surface 

 of a substrate of much lower scattering power (see 

 reference 6). 



241 



