1910] on Polarized Light and its Applications to Engineering 591 



C at right angles, these latter being capable of adjustment by 

 micrometer screws D and E, in such a manner that the measuring 

 instrument could be moved to any position with respect to the 

 specimen, and the horizontal and vertical co-ordinates determined to 

 an accuracy of two inches. 



In the photograph now shown, a block of square section is sub- 

 jected to compression stress, and the optical effects show that the 

 stress is far from uniform, although all possible precautions were 

 taken to ensure a perfectly uniform bearing. It appears to be of 

 much less intensity at the end faces than at the centre of the block. 

 The measurements confirm this, and show that there is a very con- 

 siderable end effect, tending to reduce the compression stress at the 

 centres of the end faces, and only disappearing at a moderate distance 

 away. In work of this kind where it is necessary to examine the 

 effect of the pressure of an opaque body upon a transparent one, 

 there is considerable difficulty in accurately measuring the stresses 

 very close to the boundary between the surfaces. 



The accompanying diagram (Fig. 10) of some preliminary 

 measurements indicates the general character of the distribution, in 

 which the curves denote the vertical compression stresses at various 

 distances from the end faces. 



The non-uniform character of the stress distribution, is evidently 

 due to the way in which the pressure is applied to the material under 

 test, and in this example the brass pressure plates prevent free lateral 

 displacement of the material under load. If, therefore, a more ex- 

 tensible material is interposed for transmitting the load to the 

 block, we may expect to obtain greatest stress at the centre, and this 

 is actually what occurs. 



A convenient material to interpose is a thin sheet of india- 

 rubber, and the accompanying photograph of the colour effects 

 clearly shows the widely different character of tlie stress distribution. 

 The stress at the centre line is now the greatest, and it is, moreover, 

 artificially increased 20 percent or thereabouts, for the same total load 

 owing to the action of the interposed film of india-rubber, as the 

 measurements of the accompanying Fig. 11 show. It is, moreover, 

 not a local effect confined to a small area at the ends, but exerts an 

 effect over the greater part of the block. 



The injurious effect of a lead plate, when applied to give a bearing- 

 over a block of material under test, is confirmed by this experiment, 

 and the measurements give a quantitative estimate of the increase in 

 stress intensity. 



To ensure uniform stress conditions, the experimental results 

 appear to point to the conclusion that the load ought to be applied 

 through an intermediate layer of the same material, and when this 

 is carried out, the block shows a very nearly uniform optical effect, 

 and the measurements prove that the stress intensity is nearly 

 uniform throughout. 



