1916] on Polarized Light and its Applications to Engineering 595 



section is not justifiable, and within the elastic limit it is clearly not 

 so, as is immediately obvious from the colour effects in the model, 

 where stress distribution due to a shearing force applied to a 

 rectangular section, is shown at one of the vertical cross sections. 



Commencing from the left-hand edge. Fig. 12, we observe that 

 the stress is a maximum here, and diminishes rapidly as we proceed 

 along the section until it becomes sensibly uniform, and then as 

 the right-hand edge is approached, the stress again rises rapidly, and 

 reaches its greatest intensity at the lower edge. This increase of 

 stress beyond the left-hand edge is due to the bending of the specimen 

 in the grips, and it indicates that failure will occur here. 



Measurements such as those given in Fig. 12 show that this is 

 substantially the distribution which occurs, m a specimen of rect- 

 angular section, and they give a measure of the stress at all points of 

 the section. 



Eye-Bars. 



A promising development of experimental work with polarized 

 light, relates to the design of machines and structures, especially the 

 detailed parts or components. It has already been pointed out that 

 in the majority of cases the stresses in even the simplest members are 

 so complicated as to defy exact calculation, and with the simplifying 

 assumptions usually adopted, it is necessary to allow for faulty methods 

 by using a large factor of safety. 



AVe may take a very simple example in the case of a member which 

 is merely required to transmit a pull in the direction of its length by 

 means of pins, as shown in the diagram. 



If we take a member of rectangular form bored out at each end 

 to receive the pins, then it is at once apparent that the material 

 around the pin is very highly stressed compared with the body of 

 the member, and, as we have already seen in the case of the rivet, 

 there is a compression stress of very large intensity in one region, 

 and a tension stress of considerable magnitude in another, but in the 

 main body of the member the section is excessive for the load 

 applied. We have here, in fact, the problem which confronted the 

 Deacon in Oliver Wendell Holmes' story of the wonderful " One Hoss 

 Shay " which " ran a hundred years to a day." 



" Fur," said the Deacon, " 't's mighty plain 

 Thut the weakes' place nius' stan' the strain ; 

 'n' the way t' fix it, uz I maintain, 



Is only jest 

 To make that place uz strong uz the rest." 



In practice, therefore, the superfluous material of the main 

 body is cut away, and a link is obtained with swelled ends, shaped 

 to resist in the best possible wav the stresses which come upon them. 



2 R 2 



