54 



Mi' McConnel, On Measurement 



[Nov. 26, 



the relative retardations could not be obtained by measuring the 

 well-known dark rings seen when a plate of quartz is inserted 

 between two crossed Nicols ; the light being made to pass through 

 the quartz in a convergent pencil. For this purpose Professor Lewis 

 was kind enough to lend me a polariscope and some thick crystals 

 of quartz cut perpendicular to the axis. 



It is important that the plate of quartz should be thick ; for 

 the peculiarities of quartz are only manifested when the light 

 makes a small angle with the axis, and the thicker the plate the 

 more rings will be formed within this small angle. The plate of 

 quartz I used was about an inch thick. 



The arrangement of apparatus was as follows. First a sodium 

 flame placed behind a screen with a small aperture which was 

 closed with ground glass. This was found to give the necessary 

 steadiness to the light. At a distance of about four feet was placed 

 a Nicol's prism. Then came the frame of the polariscope which 

 supported the rest of the apparatus. This consisted of a lens A to 

 make the light converge ; the crystal of quartz ; a lens B at whose 



focus the cross wires were put ; an eye lens G which rendered the 

 rays parallel ; and a second Nicol next the eye. The quartz was 

 attached to a horizontal circle on the top of the frame, fitted with 

 a vernier reading to minutes. This afforded the means of measur- 

 ing the diameters of the rings. The lens B was limited by a 

 diaphragm with a small aperture ; so as to secure that the pencil 

 of light should pass through the centre of the lens, and thus errors, 

 due to spherical aberration and the cross wires not being accurately 

 in the axis of the lens, should be avoided. When the quartz was 

 inclined the lateral displacement it produced on the light was so 

 great that it was necessary to also incline the frame of the 

 polariscope to the incident light; but this had no influence on the 

 readings. The cross wires were really thin lines ruled on glass 

 and blackened. 



