1916] on Polarized Light and its Applications to Engineering 599 



periphery of the hole, the stresses begin to equalize across the section 

 as more and more load is applied. This tendency to equalization of 

 stress beyond the yield point has also been observed in other cases 

 of a simple character, notably in beams and eccentrically loaded 

 tension members, and the value of ductility in preventing excessive 

 stresses by local yielding is demonstrated. 



The effects of overstress in brittle materials like cast-iron may 

 also be studied with glass models, and the behaviour of the latter 

 material may possibly be more representative, since its stress strain 

 properties are very like cast-iron, and, in addition, it has also a 

 crystalline structure. 



The failure of ductile materials under stress is also interesting 

 from another point of view, since the results of several investigators 

 in recent years appear to demonstrate that such materials fail 

 by shear. 



As we have seen, the colours produced are proportional to the 

 difference of the principal stresses, and, since the shear stress at the 

 same point is one-half this difference, the images projected on a 

 screen show the distribution of shear stress in a stressed model, and 

 we can determine the maximum values by inspection. Further, the 

 points where failure takes place are indicated by the transparent 

 material becoming opaque owing to the breakdown of its structure, 

 and these can be readily ascertained. A transparent model can be 

 tested to failure, and, if it is weak at any point, experiment indicates 

 the place, and in general affords some indication of the alteration 

 needed in the design to cope with the imposed load. 



It would not be difficult to supply other examples of the use 

 of optical investigation, but the cases already described are possibly 

 sufficient to show the use of polarized light in engineering problems 

 of stress and strain, and to indicate the possible utility of stress 

 pictures in other fields of applied science and industrial research. 



[E. G. C] 



WEEKLY EVENING MEETING, 



Friday, February 25, 1916. 



Sir James Crichton-Browxe, J.P. M.D. LL.D. F.R.S., 

 Treasurer and Vice-President, in the Chair. 



Professor Sir Arthur Quiller-Couch, J.P. M.A. Litt.D. 



The Commerce of Thought. 



[No Abstract.] 



