10 Prof. L. T. More on Electrostriction. 



The system of optical levers used for magnifying the ex- 

 pansion of the tubes is shown in PI. I. fig. 2. An incandescent 

 electric lamp with a ground-glass globe was clamped behind a 

 metal screen pierced with a round hole 5 millims. in diameter. 

 Across this opening fine glass fibres were fastened horizontally, 

 to serve for the movable image. The light from the lamp, 

 after reflexion at right angles by a totally-reflecting prism, 

 passed through an achromatic lens to the mirror m mounted 

 vertically on a little tripod-table. Upon reflexion from this 

 mirror it again passed through the lens and on, above the 

 prism, to a micrometer-microscope. 



The mirror (fig. 3), 1'5 cm. by 2 cms., furnished by Brashear, 

 was silvered on the front face and plane to \\. The tilting- 

 table of brass, 1*6 cm. by 1*6 cm. had three legs made of the 

 finest needle-points, one of which rested on the experimental 

 tube and the other two on the brass cap I) (fig. 1). By 

 raising or lowering the cap, the beam of light could be re- 

 flected at any desired angle. Small weights, which hung 

 from an arm below the level of the table, increased the sta- 

 bility to such a degree that the image showed no oscillation 

 from outside disturbances. The prism and lens were of the 

 best construction, and the micrometer-microscope was a new 

 instrument obtained especially for this experiment. The 

 fibres and the microscope were each placed in the principal 

 focus of the lens ; the image of these fibres in the microscope 

 was remarkably distinct and sharp. 



From the arrangement of the parts, an elongation of the 

 experimental tube caused the image of the fibres to fall in the 

 microscope. The least change in length which could be 

 observed was calculated from the following dimensions : — 



Table III.— Dimensions of Magnifying Power. 



Focal length of lens 100 cms. 



Distance between feet of tripod 1 6 mm. 



» _ 3^ ^ ,, ,, II 3 mm. 



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



A deflexion of one division of the micrometer, using tripod I., 

 is equal to a change of length in the tube of 



^00^-002 = 6xl0-mm., :■ 



and with tripod II. one-half of the above, or 3x 10~^ mm. 

 My former apparatus had a sensibility of 1*5 x 10~^ mm. ; 



