ABSTRACTS OF TECHNICAL PAPERS 631 



dence against a face of a nickel crystal, and observations were made 

 upon the diffraction beams which issued from the incidence side of 

 the crystal at various critical speeds of bombardment. It was antici- 

 pated that if the angle of incidence were made other than zero a beam 

 of electrons would be found issuing from the crystal at a series of 

 critical speeds in the direction of regular reflection, and that the series 

 of critical speeds would change with the incidence angle. This regu- 

 larly and selectively reflected electron beam which is the analogue of 

 the Bragg x-ray reflection beam has been found, and measurements 

 have been made upon it. In the x-ray phenomenon the wave-length 

 of the reflected beam at maximum intensity is related through a 

 simple formula to the angle of incidence and a dimension of the 

 reflecting crystal. This formula (Bragg's formula) does not obtain 

 in the case of electron reflection because of the refraction of the 

 electrons by the crystal. The departures from the Bragg relation are 

 used to calculate indices of refraction of nickel for electrons of various 

 speeds or wave-lengths. 



Introduction to Mathematics of Statistics. ^ R. W. Burgess. This book 

 (282 pp.) discusses the best elementary methods of statistical analysis 

 from the standpoint of a beginner who has had one year of college 

 mathematics, or some practical statistical experience and the ordinary 

 high school mathematics. "Statistical Analysis" is regarded in this 

 book as the logical process by which large masses of quantitative facts 

 may be classified, summarized, analyzed, and compared so as to yield 

 reliable conclusions. 



The topics treated include classification, formation of statistical 

 series, use of ratios and percentages in statistical analysis, meaning 

 and graphic discussion of frequency distributions, averages, index 

 numbers, measures of dispersion, trend lines, analysis of seasonal 

 variation, two-, three-, and four-variable correlation, and the elements 

 of sampling and probability. Emphasis is placed on the type of 

 statistical problems most common in the social sciences, in which the 

 data are subject to a higher degree of variability than in the usual 

 problems in physics or astronomy which require the use of the theory 

 of least squares or the Gaussian "curve of error." 



The Use of a Moving Beam of Light to Scan a Scene for Television.^ 

 F. Gray. The paper is a discussion of a method of scanning employed 

 in the television system demonstrated a year ago at the Bell Telephone 

 Laboratories. A three-dimensional subject is scanned directly by a 



1 Houghton-Mifflin Company. 



2 Journal of the Optical Society of America, Vol. 16, pp. 177-190, March 1928. 



41 



