632 BELL SYSTEM TECHNICAL JOURNAL 



moving beam of light to produce a picture current in photoelectric 

 cells. This method permits the use of a very intense transient 

 illumination and more than one large-aperture photoelectric cell to 

 collect reflected light. These two factors give a highly efficient optical 

 system for producing a picture current at a transmitting station. The 

 image seen at a distant station is the same as if light came out of the 

 photoelectric cells to illuminate the subject and a small aperture lens 

 formed an image of the subject for transmission. The television 

 system transmits only the spacial variations of brightness and not 

 the absolute brightness of the view; consequently, an additional 

 steady illumination of a subject does not affect the reproduced image. 



Maintaining High Standards in Products} E. D. Hall. This 

 article presents briefly but clearly the essential features of a method 

 of keeping before the management an accurate picture of the relative 

 quality of manufactured products. Defects are grouped into four 

 classes and given demerit grades that represent the seriousness of the 

 fault. Defects found each month are added, reduced to an average 

 value, and plotted on charts which as a reference base use the average 

 quality of the preceding five years. A method of computing averages 

 for an entire line of products is also given. 



Probability and Its Engineering Uses? Thornton C. Fry. This 

 book of 470 pages on the Theory of Probability is written from the 

 standpoint of the engineer. Its earlier chapters deal with the funda- 

 mental mathematical concepts that underlie the theory, and its later 

 chapters develop these concepts in the directions of their application 

 to traffic and trunking problems, curve fitting, and atomic physics. 



Among the subjects which receive especial emphasis are: the logical 

 standing of attempts to determine the probability of an event by trial ; 

 the physical significance of the fundamental distribution laws, such 

 as the Normal, the Binomial, and the Poisson Law; Pearson's criterion 

 for "goodness of fit"; and trunking problems. 



Differential Intensity Sensitivity of the Ear for Pure Tones.^ R. R. 

 RiESZ. The ratio of the minimum perceptible increment in sound 

 intensity to the total intensity, AE/E, which is called the differential 

 sensitivity of the ear, was measured as a function of frequency and 

 intensity. Measurements were made over practically the entire range 

 of frequencies and intensities for which the ear is capable of sensation. 

 The method used was that of beating tones, this method giving the 



1 "Manufacturing Industries," Vol. 16, No. 1, pp. 17-19, May 1928. 



3 D. Van Nostrand Company. 



3 The Physical Review, May 1928, Vol. 31, No. 5, pp. 867-875. 



