484 



Professor Silvanus P. Thompson 



[May 17, 



Fig. S. 



analyser is turned towards the left, and greenish when the analyser 

 is turned towards the right ; whereas, when I substitute the left- 

 handed slice, the tint grows greenish as the analyser is turned towards 

 the left, and reddish when it is turned towards the right. If the 

 analyser is turned through an exact right-angle, we get an extinction 

 of the yellow light, the remaining blue and red rays combining to 

 give us the purple transition tint. 



You will have noticed that the way in which we have (approxi- 

 mately) measured the angle of rotation has been first to set the 

 analyser to extinction, then to introduce the substance which has the 

 property of rotating the beam, then to turn the analyser again to ex- 

 tinction, and read off its angle. For, of course, the angle through 

 which the analyser is turned measures the angle through which the 

 plane of polarisation has been turned. 



It is possible, however, to show in the lantern something like a 

 more obvious rotation of the light by introducing between the Kicols 



a crystal star, built up of radial 



pieces of mica, twenty-four in number 



(Fig. 8). You see in the bright 



Held a white cross with black sectors 



at 45'. Or, in the dark field we have 



a black cross with vertical and 



horizontal arms, the sectors next to 



those that are black seeming dusky. 



If now I put in a quartz plate 



between the star and the analyser, 



you see the cross shift round, and 



it shows colours, because the 



rays are twisted round more 



the green, the green than 



yellow, the yellow than the 



Repeating the experiment 



the 3 "75 millimetre quartz which 



turns yellow waves round just 90', 



we get this gorgeous radiation of 



colours, and our black cross is turned 



into a yellow one. With the 7 • 5 millimetre quartz, the black cross 



is replaced by one of " transition " tint. 



The black crosses seen in certain sections of natural crystals, 

 sphseroliths, sections of stalactites, crystallisations of salicine and of 

 Epsom salts, may also be used instead of the 24-rayed star of mica. 

 But best of all I find to be the beautiful black cross which is seen by 

 polarised light in the prepared crystalline lens taken from the eye of 

 a fish. You notice how, when the fish-lens is projected and the quartz 

 introduced, the cross turns round. 



This is, however, a rough-and-ready way of displaying the rotation, 

 and it is of vast practical importance that precise methods of measur- 

 ing the angle of rotation should be available — of vast importance, 



blue 



than 



the 



red. 



with 



Mica disk of twenty-four rays, 

 showing black cross in the dark field. 



