Refraction \Re*frac"tion\ (r?*fr?k"sh?n), n. [F. r['e]fraction.]
1. The act of refracting, or the state of being refracted.
2. The change in the direction of ray of light, heat, or the
like, when it enters obliquely a medium of a different
density from that through which it has previously moved.
Refraction out of the rarer medium into the denser,
is made towards the perpendicular. --Sir I.
Newton.
3. (Astron.)
(a) The change in the direction of a ray of light, and,
consequently, in the apparent position of a heavenly
body from which it emanates, arising from its passage
through the earth's atmosphere; -- hence distinguished
as atmospheric refraction, or astronomical refraction.
(b) The correction which is to be deducted from the
apparent altitude of a heavenly body on account of
atmospheric refraction, in order to obtain the true
altitude.
Optometry \Op*tom"e*try\, n.
1. (Med.) Measurement of the range of vision, esp. by means
of the optometer.
2. As defined (with minor variations) in the statutes of
various States of the United States:
(a) ``The employment of subjective and objective
mechanical means to determine the accomodative and
refractive states of the eye and the scope of its
function in general.''
(b) ``The employment of any means, other than the use of
drugs, for the measurement of the powers of vision and
adaptation of lenses for the aid thereof.
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