POLARISED LIGHT 381 



quarter- wave plate. The result is that a plate cut to exhibit 

 one axis only of a bi-axial, shows one spiral figure surrounding 

 this axis. A plate cut to show both axes, exhibits a few turns 

 of a spiral round each axis, after which the two spirals em- 

 brace each other, and proceed concentrically round the whole 

 field. With a low-angled bi-axial cut thin, in highly conver- 

 gent light, the two spirals enwrap each other nearly from the 

 first. And finally, in a uni-axial the two separate spirals are 

 visible, showing that both elements of the bi-axial remain and 

 are combined in what is simply a limiting case. By applying 

 the arrangement to Mitscherlich's experiment with a heated 

 sslenite, the gradual drawing in of the figure, but preservation 

 of the two spirals through all, can be simply demonstrated. 



A quartz plate alone in highly convergent circularly 

 polarised light, projects a double spiral, as pointed out by 

 Mr. Airy. The above experiments prove that this is owing to 

 its peculiar properties enabling it to show its own spirals as a 

 uni-axial crystal. 



A column of fluid of adequate rotary power, such as a 

 column of oil of lemons 20 cm. long, employed with a crystal 

 instead of the quartz plate, will exhibit exactly the same 

 phenomena, thus affording proof that the molecular con- 

 stitution of the fluid resembles that of the quartz. The 

 polariscope I have described projects through the column of 

 fluid, crystal, and convergent lenses, &c., brilliantly and with 

 ease. 



It will be very interesting to point out, that by thus 

 modifying the polarisation in different ways, rays of the same 

 convergence can produce with the same plate of crystal, either 

 rings with brushes, rings dislocated, unbroken rings without 

 any brushes, or spirals. 



222. Artificial Crystals. It has been seen that a plate 

 of crystal circularly polarised, roughly represents the rotary 

 phenomena of quartz. Keusch, by employing a preparation 

 built up of many thin films of mica successively rotated on 



