324 



Dr. J. Kerr's Experiments on the 



My model of this instrument is national property : it was 

 constructed for me by M. Lutz with his usual skill, and goes 

 far to realize the inventor's very exacting conception. With 

 the instrument as it stands, it is not very difficult to measure 

 a difference of retardations of the two pencils, true probably 

 to the hundredth of an average wave-length, that is to the 

 five millionth of an inch of path in air. Under very favour- 

 able conditions (careful setting of the pieces, moderate range 

 of effect, absence of tremors, &c), the error of a single deter- 

 mination of effect should not exceed the twentieth part of an 

 average wave-length. 



3. Designation of the two acting sets of rays. — The two sepa- 

 rated systems of rays in the interference refractor, between 

 the half-wave plate and the second spar, will be designated 

 briefly, throughout this paper, as the pencils V and H, their 

 planes of polarization being thus distinguished as vertical 

 and horizontal respectively. 



Proposition I. 

 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. 



4. The discovery of this fundamental proposition was 

 made with a bent plate and the interference refractor. The 



i 



Fig. 2. 



i 



glass plate shown in the diagram is 10 to 12 inches long, -J to 

 | thick, 1 \ to 2 wide. Its two sides are ground flat, sensibly 

 parallel to each other, and perpendicular to the plate-faces. 

 The three small semicircles in the diagram are bearings of 

 ebony, semicylindrical and equal, through which the bars of 

 the screw-press act upon the plate. The small rectangle on 

 the upper half of the plate* is a slit-screen of paper, which 

 is pasted on the glass in a position chosen carefully with the 

 help of the polariscope, so that the slit lies along the middle 

 * The plate stands in the experiments with its length vertical. 





