BIBLIOGRAPHY 351 



rays in passing through matter is expressible by the relation 



v% — v e 4 = ad 



where v; is the velocity of the incident and v e the velocity of the emerging 

 electron after passing through a thickness d, and a is a constant depend- 

 ing on the density of the absorbing substance. Later Terrill [1923] 

 showed that a = 5.05 X 10 42 p, where p is the density of the material. 



The maximum depth of penetration into a photographic emulsion, 

 here assumed to have a density comparable to that of gelatine,* is there- 

 fore approximately d = v t 4 X 10 -43 cm. 



If the electrons reach the film with a velocity determined by the 

 accelerating voltage V, the depth of penetration 



d = 1.2 X 10~ 12 7 2 cm 



If the photographic emulsion is 10~ 3 cm thick, 30-kilovolt electrons 

 would penetrate this film completely, producing a very effective pho- 

 tographic action, since the developed blackening of a photographic 

 emulsion depends on the concentration of the electron beam and on the 

 square of the accelerating voltage. 



BIBLIOGRAPHY 



1912 Whiddington, R., Proc. Roy. Soc. London, A86, 370. 



1923 Terrill, H. M., Phys. Rev., 22, 101. 



1925 Wood, A. B., /. Inst. Elec. Engrs. (London), 63, 1046. 



1926 Busch, H., Ann. Physik, 81, 974. 



1929 George, R. H., J. Am. Inst. Elec. Engrs., 48, 534. 



1930 Knoll, M., H. Knoblauch, and B. v. Borries, Elektrotech. Z., 61, 966. 



1931 Davisson, C. J., and C. J. Calbick, Phys. Rev., 38, 585. 



1932 Knoll, M., and E. Ruska, Z. Physik, 78, 318. 



1933 Zworykin, V. K., /. Franklin Inst., 215, 535, or J. Inst. Elec. Engrs. (London), 

 73, 437. 



1934 Bruche, E., and O. Scherzer, Geometrische Eleclronenoptik, Springer, Berlin. 

 1934 Marton, L., Bull. acad. roy. Belg., CI. Sci., 20, 439; also 21, 553 and 606 



(1935); 22, 1336 (1936); 23, 672 (1937). 

 1934 Marton, L., Nature, 133, 911. 

 1934 Ruska, E., Z. Physik, 87, 580; 89, 90. 



1934 Zworykin, V. K., /. Franklin Inst., 217, 1. 



1935 Beeching, R., Electron Diffraction (elementary de Broglie theory), Methuen 

 and Company, London. 



1936 Busch, H., Z. tech. Physik, 17, 584. (Electron optics symposium.) 



1937 Krause, F., Naturwissenschaften, 25, 817. 

 1937 Law, R. R., Proc. Inst. Radio Engrs., 25, 954. 



1937 Martin, L. C, R. V. Whelpton, and D. H. Parnum, J. Sci. Instruments, 

 14, 14. 



* Gelatine has an average density of 1.27 grams /cc. 



