Definition
Swing-in index shades sit between the index mirror and the horizon mirror to attenuate the body, while horizon shades reduce glare from a bright sea surface. Never observe the sun without adequate index shading, since the concentrated reflected image can cause permanent retinal damage. Polarising filters offer variable attenuation. Choose the lightest combination that still gives a crisp limb, because excessive filtering blurs the edge and degrades the sight.
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