37 6 OPTICAL PROJECTION 



colour is visible ; but with a bi-quartz the effect is generally 

 very evident with a moderate current. Thompson's mica 

 sectors (fig. 200) may also be used. 



Dr. Kerr's experiment, showing that plane-polarised light 

 is rotated if reflected from the polished pole of a magnet, 

 is too sensitive for successful demonstration, unless with 

 apparatus of the highest class and the arc light. 



220. Rotation of Common Light. If the light employed 

 is powerful enough to exhibit interference -bands on the screen 

 with Fresnel's bi-prism ( 189) this can be exhibited after the 

 manner of Profs. Abbe and Sohncke. It is only necessary to 

 cover one-half of the bi-prism with a plate of left-handed, 

 and the other with a plate of right-handed quartz, of 1-88 

 mm. in thickness. This thickness rotates yellow light 45, 

 and therefore if quartz rotates common light at all, the two 

 interfering rays are brought into orbits differing by 90 in 

 azimuth, when they cannot interfere. It is demonstrated 

 that this is so, because the bands vanish accordingly. 



221. Ring and Brushes in Crystals. For use with the 

 simpler forms of lantern polariscopes, like fig. 186, plates 



of crystals are cut 

 transversely to their 

 optic axes, and 

 mounted in wooden 

 sliders as repre- 



FIG. 207. .... _ 



sented in fig. 207. 



By having a frame made of the usual 4x2^ size, consisting ol 

 two thin metal plates with a circular aperture in the centres, 

 separated by a strip of wood along the top and bottom edges, 

 and leaving a space for the small slide as a centre strip 

 between, this can be placed in the usual stage, when it will be 

 seen that the plate of crystal (unless it be a circularly-polar- 

 ising one) has no double refraction at all in this direction ; the 

 dark or light field, in the parallel beam of plane-polarised 

 light, remains as it was before the crystal was inserted, 



