Strain along the Lines of Force. b'89 



desired angle. Small weights which hung helow the level of 

 the table from an arm increased the stability to such a degree 

 that the image showed no oscillation from outside disturbances. 

 The prism and lens were both of the best construction, and 

 the microscope was a new instrument obtained especially for 

 these experiments. The platinum wires and the microscope 

 were each placed in the principal focus of the lens ; the 

 image was then very sharp and distinct. The least change 

 in length which can be observed is calculated from the 

 following dimensions : — 



Dimensions for Magnifying Power. 



Focal length of lens 100 cm. 



Distance between feet on tripod I 6 mm. 



Distance between feet on tripod II 3 mm. 



One division on micrometer of microscope. 0'002 mm. 



A deflexion of one division of the micrometer, using 

 tripod I, is equal to a change of length in the condenser 



of ^ — tttht\ x 0-002 = 6 x 10~ 6 mm., and with tripod II 

 2 x 1000 l 



3 X 10 -6 mm. This minimum deflexion could be easily 



observed. In compiling readings, they were reduced to the 



basis of 6 X 10~ 6 mm. for one division, whichever tripod was 



used *. 



The criticisms made on this type of apparatus have been a 

 surprise to me, as I supposed such a standard form would 

 impress all with its capabilities for delicacy and accuracy. 

 In the first place, it is as much a form of optical device as the 

 interferometer, and it has just about the same refinement as 

 all optical devices, namely, its limit involves the wave-length 

 of light just as interference methods do, and on the other 

 hand it seems to me better adapted to the experiments under 

 discussion. 



Duriug preliminary experiments made to test the insulation 

 and the dielectric strength of the condenser, I inserted a sheet 

 of soft rubber between the lower surface of the top plate of 

 the frame and the upper plate of the dielectric, using no oil. 

 With this arrangement I noticed small deflexions of the 

 mirror, but in every case the deflexion showed an elongation 

 of the dielectric, and not the contraction the theory calls for. 

 This elongation was also much smaller than the theories of 

 electrostriction demand. I thought at first that it was due to 



* I wish here to express my thanks and indebtedness to Mr. P. B. Evens, 

 the University mechanician, for the skill and patience he has exercised in 

 the making of all this apparatus which made these experiments possible. 



