1889.] 



on Optical Torque. 



487 



Another suggestion, due to Senarmont, is to use two sets of super- 

 posed wedges of riglit- and left-handed quartz. Such you now see 

 before you. Instead of starting with extinction you start with coinci- 



FiG. 10. 



->-. 



Direct-vision prism. A, wide-angled prism of Jena glass ; C, cinnamic eth( 



-5-K 



V 



dence between the upper and lower set of bands. Any rotation of the 

 light shifts the bands, one set moving to left, the other to right. 

 By turning the analyser through an equal angle coincidence is again 

 obtained. 



Another method, used by Wild in his polaristrobometer, is to 

 produce the phenomenon known as SavaTt's bands (due to the intro- 

 duction of two crossed slices of quartz cut at a particular angle). 

 The bands disaj)pear when the analyser is set in a particular direction. 

 Anything that twists the plane of polarisation causes them to reap- 

 pear ; but they again fade out when the analyser is turned through 

 an equal angle. 



There is another method in exact polarimetry, due to Soleil, in 

 which the optical torsion due to the sugar is counterbalanced or 

 compensated by introducing a pair of sliding wedges of quartz of 

 the opposite rotation. This device is known as a "compensator." 

 By sliding the quartzes over one another a greater or less thickness 

 of quartz is introduced at will. But I must not stop to illustrate this 

 elegant device. 



Yet one other method must be mentioned, and this is certainly 

 the most preferable. It consists in aiding tlie eye to recognise with 

 precision a particular degree of extinction, by the device, first suggested 

 in 1856 by Pohl, of covering a portion of the visible field with some- 

 thing which slightly alters the initial plane of polarisation, so that 

 complete blackness is not obtained at once over both parts of the 

 field. A common device is to cover half the field with a slice of some 

 thin crystal — mica or quartz — so that only one half can be perfectly 

 black at any instant. As an example, here is the field covered half 

 over with a plate of mica of the thickness known as half-wave. The 

 result is that when one half of the field is black the other is light. 

 Adjust the analyser now to equality. Now introduce something that 



