1889.] on Optical Torque. 485 



because in several large industries this optical process is applied as a 

 method of rapid analysis. I have named a solution of sugar as being 

 an " active " substance. In the industry of sugar-refining, as in that 

 of brewing, the strength of sugar in the liquids is dii-ectly measui'ed 

 by measuring its optical effect. Consequently there has been deve- 

 loped a special instrument, the polarimeter, for this express purpose. 



I have here examples of several practical forms of polarimeters ; 

 there are diagrams of several more uj)on the walls. 



The problem of finding the best polarimeter naturally leads to 

 the inquiry what special means there are for making the observation 

 of the angle more precise than by merely observing the extinction of 

 the light, its restoration when the active substance is interposed, and 

 the subsequent renewal of extinction when the analysing prism is 

 turned. 



Biot considered that much greater a<?curacy could be attained by 

 watching for the restoration of the sensitive tint than by watching for 

 the mere restoration of extinction of the light. Accordingly we will 

 use the plate of quartz 7 • 5 millimetres thick, giving the purple tint, 

 to enable us to measure the rotation produced by the tube of sugar 

 solution which is now inserted in the beam of polarised light. You 

 notice how the tint has changed. But I have only to turn the 

 analj'ser to an amount equal to that to which the light has been 

 twisted by the sugar, and again I obtain the sensitive transition tint. 



The eye is not always, however, aliv^ to minute changes of colour 

 in a single coloured patch ; it much more readily distinguishes a 

 minute difference between two tints when both are present at once. 

 Hence Soleil devised the well-known biquartz arrangement, consisting 

 of two pieces of crystal, equal in thickness, but possessing opposite 

 rotations. You will notice how the slightest inaccuracy in placing 

 the analyser causes the two halves of the field to differ in tint. This 

 is especially marked when the tint chosen is the transition purple. 



It will be convenient here for me to refer to some researches, not yet 

 published, which I have made, as to the various orders of transition 

 tints, with the view of ascertaining which of them is the most sensitive 

 — which of them, in fact, shows the greatest change of tint for the 

 smallest amount of rotation. Eeference to the diagram on the wall 

 displaying Xewton's tints will make clear what I mean by the transi- 

 tion tints of the several orders. The tints obtained from quartzes of 

 varying thicknesses may be considered as approximately identical 

 with the tints of Newton's rings, provided we remember that the air- 

 film which gives any particular tint in Newton's rings is about 

 1/300,000 part as thick as the quartz which yields the corresponding 

 tint in the polariscope. Better far than any painted diagram, because 

 richer and purer, are the tints now thrown upon the screen by intro- 

 ducing into the field a thin wedge of selenite, displaying the whole of 

 the colours of the first three orders of Newton's scale. You will 

 notice the successive recurrence of purple tints, both in the 

 colours seen in the bright field, and in those seen in the dark field. 



