Abstract: Dihydric phenol suitable for the preparation of polycarbonates are represented by the following formula: ##STR1##

Abstract: A resin material for optical instruments comprising a polycarbonate having the repeating units represented by the following general formula: ##STR1## wherein X represents ##STR2## has an extremely small birefringence particularly to oblique-incident lights.

Abstract: A polycarbonate polymer containing repeating units represented by the following general formula ##STR1## wherein: X is selected from single bond, ##STR2## wherein: R.sup.1 and R.sup.2 are each independently selected from hydrogen atom; alkyl radical of 1 to 6 carbon atoms; cycloalkyl radical of 5 to 6 carbon atoms; ##STR3## wherein p is an integer having a value of from 0 to 3; phenyl radicals substituted at para position by a substituent selected from alkoxy radicals of 1 to 5 carbon atoms, phenoxy radicals, or halogen radicals; or p-biphenylyl radical, --CH.sub.2).sub.q wherein q is an integer having a value of from 2 to 10; or ##STR4## wherein r is an integer having a value of from 4 to 8, and having a reduced viscosity [.eta.sp/c] of at least 0.2 dl/g as measured in methylene chloride at a concentration of 0.5 g/dl at 20.degree. C.Such polycarbonates exhibit excellent optical properties such as high birefringence, and are useful as materials for optical devices.

Abstract: Polyformal resins having a repeating unit represented by the general formula: ##STR1## wherein the symbols are as defined in the appended claims and an intrinsic viscosity at 20.degree. C. as a methylene chloride solution of at least 0.2 dl/g, and a process for efficiently producing the polyformal resins. The present polyformal resins are excellent in heat resistance and optical properties, and thus are useful as materials for production of optical instruments, for example.