1889.] on Optical Torque. 483 



table of data to whicli I invite your attention states this amount for 

 the different colours. 



Optical Torsion Produced by Plate of Quartz. 



1 millimetre. 3-75 millimetres. 



If we use a piece of quartz so thick that it rotates any 

 particular tint just 90^, that tint will be cut off by the crossed 



analyser, and all others will — in greater or less proportion be 



transmitted, so that the resulting tint will be complementary to 

 that cut off. For example, a slice so thick as to twist yellow 

 waves round 90° must be just 3*75 millimetres thick. (I may 

 remark, for the benefit of those who think it easier to express this 

 exact thickness in fractions of a British inch, that the quartz which 

 rotates yellow light 90° must have a thickness equal to one-ei»hth, 

 plus three-sixteenths of an eighth, plus one sixty-fourth of an eighth of 

 an inch.) When such a quartz is placed between the crossed Nicols 

 the light shown is yellow ; but if placed Between parallel Nicols (i. e. 

 in the bright field) it shows a rich j)urplish-violet colour, the comple- 

 mentary of the yellow. This particular tint Biot found to be 

 excessively sensitive, the smallest inaccuracy in adjustment between 

 the prisms at once producing a change, the colour appearing too red or 

 too blue, according to the directions in which the analyser has been 

 turned out of exact adjustment. This tint is accordingly known as the 

 " transition tint " or " sensitive tint," its accurate definition being due 

 to the fact that the human eye is more sensitive to the presence or 

 absence of the complementary yellow than to any other tint in the 

 whole spectrum. If we take, however, a quartz plate twice as thick 

 as this — namely, 7J millimetres thick — it will give the yellow light a 

 torsion of 180°. Hence this thickness gives the purple transition tint 

 in the dark field, and yellow in the bright field. A quartz plate 11^ 

 millimetres thick gives again a transition tint in the bright field. I 

 shall recur presently to the question of the transition tints of the 

 several orders. 



One of the familiar facts in this subject is that there are two kinds 

 of quartz crystals, optically alike in every other respect, differinf^ 

 only in this, that one kind produces a right-handed twist, the other 

 kind a left-handed twist. All the pieces of quartz I have so far em- 

 ployed are right-handed specimens. I now introduce two small 

 slices of crystal, each 3 -75 millimetres thick, giving the yellow tint 

 when the Nicols are exactly crossed, but you will notice that when 

 we are using the right-handed crystal, the tint grows reddish as the 



