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The study of physical characteristics of earth and the distribution and effects of life on earth, especially that of humans.

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Lens blank

Eyewear; Optometry

Piece of material, usually preformed, for the making of a lens at any stage before completion of the surfacing processes.

Horizontal centre line

Eyewear; Optometry

Line located at equal distance from the two horizontal tangents of the boxing system.

Infrared radiation

Eyewear; Optometry

Optical radiation with a wavelength longer than that of visible radiation.

Illuminated magnifier

Eyewear; Optometry

Magnifier incorporating illumination.

Focal length

Eyewear; Optometry

Distance in an optical system from the focal point to the corresponding principal point, measured along the optical axis (axis of symmetry).

Fixed-location stimulus perimeter

Eyewear; Optometry

Perimeter which utilises test stimuli that are at permanent locations on the background.

Fixation target

Eyewear; Optometry

Target used to locate the point where the patient should look during testing.