a Dielectric in an Electrostatic Field. 201 



the tubes, prevented the dissolving of the wax. Of different 

 cements (such as yellow lead dissolved in glycerine, plaster 

 of Paris, shellac) sealing-wax held well and was convenient 

 to manipulate. 



The magnifying arrangement of the deflexion consisted of 

 a system of optical levers (fig. 3). The light from a Wels- 

 bach burner was placed behind a metal screen pierced with a 

 round hole one centimetre in diameter ; fine glass fibres were 

 stretched horizontally across this opening. The light, after 

 being deflected at a right angle by a totally reflecting prism, 

 passed through an achromatic lens to a mirror mounted on 

 a three-legged table and then, upon reflexion, back again, 

 through the lens and on past the prism either to one side or 

 above. The lens had a focal length of a metre; as the screen 

 was at that distance from it, the image of the fibres was also 

 at a distance of a metre. The mirror, in position, is shown 

 in fig. 1. Dimensions are in centimetres. As arranged, a 

 vertical displacement of the image accompanied a change in 

 the length of the glass tube. 



The tilting-table and mirror (fig. 2) consisted of a small 

 brass plate, 1*7 centim. square, with three legs made from the 

 finest needle-points. The mirror was 2*5 centim. high and 

 1*4 centim. in width, and was silvered on the face and back. 

 The effective distance between the needle-points was 6 millim. 

 The image of the fibre was finally observed in a cathetometer 

 microscope provided with a vertical motion micrometer screw. 

 One division measured a deflexion of 1/200 millim. 



One division of the micrometer is equal to a change of 

 length of 



X — K7 r = 1*5 X 10 -5 millim. 



2 x 1000 200 



The microscope could be set to a quarter of a division. In 

 using such a system it is advantageous to increase the stability 

 of the mirror by lowering the centre of gravity of the tilting- 

 table considerably below the needle-point. Without such an 

 arrangement it is very sensitive to external vibrations, but by 

 passing over the top of the table a yoke carrying leaden 

 weights of 20 grams, all vibrations are prevented and even 

 hard jars disturb the image but for an instant. The image 

 during observing was perfectly steady, and after a deflexion 

 returned to the original position accurately. 



To adjust the position of the reflected ray of light, a solid 

 brass cap was screwed into the upper end of the inner brass 

 cylinder (fig. 1), the top extending out so that it left a space 

 of about a millimetre between it and the glass tube. One leg 



