ELECTRON MK.KOSCOPY 



be remembered that some crystals may con- 

 tribute more than one spot to a particular 

 ring of the diffraction pattern and if the 

 aperture is a complete annulus, more than 

 one image of each crystal may be formed. 

 Because of aberrations, the images will not 

 necessarily coincide. For this reason it is 

 usually better to use a 60° or 90° arc of an 

 annulus. Often the particles of a particular 

 constituent possess some peculiar feature 

 which enables them to be identified in the 

 bright -field micrograph once the connection 

 between the feature and the particular con- 

 stituent has been established by selected 

 diffraction microscopy. 



REFERENCES 



1. Hall, C. E., "Dark-field electron microscopy. 



I. Studies of crystalline substances in dark- 

 field." J. Applied Physics, 19, 198 (Feb., 

 1948). 



2. Rang, 0. and Schluge, H., "Dunkelfeld- 



Mikroskopie mit definierten gitter-reflexen." 

 Optik, 9, Heft 10, p. 463 (1952). 



3. Rang, O. and Schleich, F., "Elektronenmik- 



roskopische Dunkelfeld-Abbildung als Mittel 

 zur Identifizierung kleiner Kristalle." Z. 

 Phys., 136, 547 (1954). 



4. Talbot, J. H., "Identification of minerals 



present in mine dusts by electron diffraction 

 and electron microscopy." Proc. Stockholm 

 Conference on Electron Microscopy," p. 353, 

 1956. 



5. Talbot, J. H., "A method of determining the 



distribution of crystalline substances in 

 electron microscope specimens." Paper pre- 

 sented at the Annual Conference of the South 

 African Institute of Physics, July 1957 (to be 

 published). 



6. Scott, Robert G., "Structure of spherulites as 



revealed by selected area electron diffraction 

 and electron microscopy." J. Applied Phys- 

 ics, 28, 1089 (Oct. 1957). 



J. H. Talbot 



SNOW CRYSTAL NUCLEI 



Snow Crystals and Ice Crystals. In the 



field of physical meteorology, the electron 

 microscope is used for the studies of nuclei 

 of snow crystals, fog and clouds, of the sur- 



face nature and the growth of snow crystals, 

 and of atmospheric aerosols. Snow crystals 

 are solid precipitations which are observed 

 on the earth's surface. They are born at a 

 high altitude and grow into various forms 

 while falling through the atmosphere. I'he 

 factors (13) that influence snow crystal types 

 are mainly air temperature and the humid- 

 ity at which the crystal grows. The initial 

 stage of a snow crystal can be seen in the 

 cirrus cloud. This small crystal is called an 

 ice crystal. In the process of formation of a 

 snow crystal in the free atmosphere, the ice 

 crystal is first formed. An ice crystal may be 

 formed by the spontaneous transformation 

 of a supercooled water droplet under the 

 threshold temperature for nucleation, by 

 the direct condensation of water vapor on a 

 solid nucleus or as the result of natural and 

 artificial seeding. After nucleation, ice crys- 

 tals grow in an atmosphere of water vapor 

 supersaturated with respect to ice. 



Method of Making Specimens. Snoio 

 crystal nuclei. The difficulty of this experi- 

 ment lies in making certain that the image 

 obtained in the electron micrographs is the 

 nucleus of snow. For this purpose, great care 

 must be taken in locating the center of a 

 snow crystal in the field of the mesh. Also, 

 since there are many aerosols in the free 

 atmosphere, it is desirable to choose a spot 

 where the natural dust is at a minimum. 



This writer undertook research in Hok- 

 kaido and in Upper Michigan where the 

 aerosols were at a minimum in the midwin- 

 ter. The specimens of snow-crystal nuclei for 

 the electron microscope were prepared in an 

 igloo, i.e., a snow cavern made for this pur- 

 pose. The temperature in the igloo was be- 

 tween — 5 and — 15°C, and snow crystals 

 could be handled without any fear of melt- 

 ing. A long and slender wooden piece re- 

 sembling a match stick was made and 

 broken into two pieces. The whisks of the 

 broken end were convenient for picking up 

 a snow-crystal without melting it because 

 of the thermal insulation of the stick. 



254 



