374 



OPTICAL PROJECTION 



small, and really more suitable for the polarising microscope. 

 Cinnabar and periodate of soda are among the best. 



217. Mica Registers of Rotation. It will be obvious that 

 a bi-plate of mica circularly polarised, as previously described, 

 may be used exactly as a bi-quartz. But far more effective 

 as a screen demonstration is a mica preparation devised by 

 Prof. S. P. Thompson, consisting of twenty-four sectors, in 

 each of which the principal axis or polarising plane is radial 

 to the circle. It is obvious that when the analyser is crossed, 

 the horizontal and vertical sectors must show a black cross, 



while the others show gradual 

 transition towards the full 

 depolarising effect of the film 

 at 45. Hence the best thick- 

 ness is either half-wave or 

 \\ waves, as this gives the 

 greatest transition from dark 

 to light. With the black 

 cross vertical, if now any 

 substance exerting rotary 

 power be introduced, the 

 cross is conspicuously rotated, 

 showing vividly what Prof. 

 Thompson calls the optical 'torque' or torsion of the beam. 1 

 Of course the rotated cross becomes more or less coloured, 

 owing to the differing rotation of the various colours, but the 

 effect is none the less simply evident. 



Any large crystallisation showing a ' rose ' with rotating 

 black cross, may be used in a similar manner. 



218. Rotation in Fluids. Many fluids may be used to 

 demonstrate this, in a tube about 20 cm. long and two inches 

 diameter, with flat glass ends screwing on. This tube should 

 be arranged to rest in two supports fixed on a base-board 

 which can be dropped into the polariscope between the 



l See Proc. Boy. Inst. xii, 474, or Nature xl. 232, 257. 



PIG. 206. Thompson's Rotation Register 



