[ 275 ] 



XXXII. Measuring Polariscopes. 

 By Professor W. Grylls Adams, M.A., F.R.S* 

 [Plate VIII.] 



SOME four years ago the description of a new measuring 

 polariscope was communicated by the author to the Phy- 

 sical Society (see ' Proceedings/ vol. i. p. 152), in which the 

 advantages gained are an extensive field of view combined with 

 accurate means of measuring the rings and the separation of 

 the optic axes in biaxal crystals. The peculiarity of the in- 

 strument consists in the arrangement of the two central lenses, 

 one on each side of the crystal. These two lenses are plano- 

 convex, very nearly hemispheres, and, with their flat surfaces 

 inwards, form the two sides of a box to hold the crystal im- 

 mersed in oil or a liquid ; they are so placed that their convex 

 surfaces form portions of the same spherical surface. The 

 crystal is placed in the box at the centre of curvature of the 

 spherical surfaces of the two lenses. 



Two instruments have since been made on this principle 

 with certain important modifications. In one, made by Mr. 

 Tisley for horizontal projection (Plate VIII. fig. 1), the pola- 

 rizer is a Nicol's prism capable of giving a clear parallel 

 beam of polarized light 2\ inches in diameter : the middle 

 portion of the instrument (i. e. the box with the two equal 

 central lenses for its two opposite sides) has an opening at the 

 top, into which the crystal to be measured is inserted, and is 

 adjusted to its right position by a cup-and-socket motion. 

 When the angle between the optic axes is to be measured, the 

 instrument is placed with its axis horizontal, the crystal is 

 placed with the plane of the optic axis vertical, the box and 

 crystal together are then turned about a horizontal axis at 

 right angles to the direction of the axis of the instrument, i. e. 

 at right angles to the plane of the optic axes : thus either of 

 the optic axes of the crystal may be made to coincide with 

 the centre of the field of view, where the spider-lines cross one 

 another, the angle through which the box is turned being 

 measured to minutes by means of a circle attached to it and 

 a vernier attached to the fixed stand supporting the instru- 

 ment. 



A table-polariscope on the same principle has been made by 

 Herr Schneider of Vienna, into which several important mo- 

 difications have been introduced. A section and view of the 

 instrument are shown in figs. 2 and 3. 



The light falls on a plane mirror A, and is reflected into the 



* Communicated by the Physical Society, having been read at the 

 Meeting on June 28. 



