@article{ref1,
title="Trifocals, their theory and application",
journal="American journal of optometry and archives of American Academy of Optometry",
year="1947",
author="Marks, R.",
volume="24",
number="8",
pages="359-367",
abstract="<p>[Abstract unavailable]</p><p>Language: en</p>",
language="en",
issn="0002-9408",
doi="",
url="http://dx.doi.org/"
}