The pitch of the mixture MBBA :CC=22.7 :77.3 (by weight) at different tempera-lures ranging from 37℃ to 55℃ is determined by method of Bragg scattering using 6328 A He-Ne laser beam. From the present measurement and the published data on other materials, we propose an empirical formula for the temperature dependence of the pitch of cholesterics, namely
p=p0-b/(T-T0),
with the pitch p<0 for levorotatory materials and p>0 for dextrorotatory materials. This formula is well suited not only for...