486 Prof. C. A. Carus Wilson on the Influence of 



It has already been pointed out* that " Neither Neumann 

 nor Maxwell seems to have remarked that the difference of the 

 velocities of the ordinary and extraordinary rays depends 

 solely on the maximum slide of planes perpendicular to the 

 wave-front." 



An important work on this subject is found in a paper by 

 Dr. John Kerrf. He establishes the fact that " If a plate of 

 glass, compressed or extended in one direction parallel to its 

 faces, be traversed normally by two pencils of light, which 

 are polarized in planes respectively parallel and perpendicular 

 to the direction of strain, then both pencils are retarded by 

 the strain in the case of compression, and both are accelerated 

 by the strain in the case of tension." Also that " strain- 

 generated retardations, absolute as well as relative, are sensibly 

 proportional to the strain," thus confirming Wertheim's 

 results. 



Dr. Kerr employs in his experiments a bent glass beam, 

 doubly supported and centrally loaded, having the ratio of 

 span to depth J of 8*4 to 1, and assumed to obey the Ber- 

 noulli-Eulerian theory. 



I would draw attention to the disagreement between the 

 results arrived at by M. Neumann and Dr. Kerr, the former 

 stating that the velocity of light in a medium is increased by 

 compressing it, while the latter states that the velocity is 

 diminished. 



Dr. Kerr examined a beam having a span equal to 8'4 

 depths, and at a point where the surface-loading effect would 

 be least; whereas M. Neumann examined a beam — span to 

 depth ratio not stated — immediately under the load. 



I can only attempt to account for the discrepancy by 

 pointing out that if the span is diminished to less than four 

 depths, the elements of glass that M. Neumann assumed to 

 be in a state of squeeze are actually, as will be shown later, 

 in a state of stretch. 



The instrument with which the following experiments were 

 made consists of a steel straining-frame in which the beam 

 to be examined is placed ; the beam rests — for flexure — on 

 two steel rollers, and is loaded by a micrometer-screw which 

 bears on a third central roller. The base of the frame is 

 divided, from the centre, in divisions of 2 millim. so that the 

 supports can be set for any required span. A micrometer- 

 screw is placed in the base of the frame opposite the load, so 

 that deflexions can be measured to one ten thousandth of an 



• Hist, of Elasticity, vol. i. p. 643. 

 t Phil. Mag. October 1888. 

 X AccordiDg to the figure. 



