trifocal, a. and n.
(traɪˈfəʊkəl)
[f. tri- + focal a.]
A. adj. † a. (See quot.) Obs.
| 1826 J. I. Hawkins in Repertory Patent Inventions III. 386 When three pairs [of glasses] are worn in one frame..which I denominate trifocal spectacles, the opening should be a circle of one inch diameter. |
b. Of a lens: having three parts with different focal lengths. Of spectacles: having such lenses.
| 1921 Amer. Jrnl. Ophthalmol. IV. 406/2 The writer does not claim that every presbyope should have a pair of trifocal spectacles. 1928 S. Duke-Elder Pract. Refraction xxiii. 343 For some purposes tri-focal lenses are advocated. 1969 K. Vonnegut Slaughterhouse-Five v. 93 She was wearing tri-focal lenses in harlequin frames. |
B. n. A trifocal lens; usu. pl., trifocal spectacles.
| 1899 J. Thorington Refraction xi. 283 Trifocals.—Occasionally, a patient is not content with bifocals, but will demand a focal point somewhere between infinity and his working distance. 1946 Berens & Zuckerman Diagnostic Examination of Eye i. xii. 319 (caption) Double⁓segment panoptik trifocal for sculptors. 1962 L. S. Sasieni Optical Dispensing vii. 172 Where the gardener does not need a distance correction, the best combination may be reading and weak intermediate. A trifocal is the obvious alternative. 1973 R. Hayes Hungarian Game xxiv. 147 He was a pallid man with eyes that were weirdly distorted by trifocals. |