Eyesight, and how to care for it / edited by George Black.

Date:
[1888]
Catalogue details

Licence: Public Domain Mark

Credit: Eyesight, and how to care for it / edited by George Black. Source: Wellcome Collection.

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    yellow, and is pleasantly modified by blue, which ab- sorbs the yellow rays. Cheap protecting spectacles are often made of poor glass, and contain flaws which produce an unpleasant distortion of objects seen through them. Coquille, or shell-glasses (those shaped like a watch-glass), if their surfaces are not parallel, may act as weak lenses to a noticeable and even annoying degree. To de- termine if a professedly plain glass is free from refrac- tion, hold it at some distance from the eye, and look through it at a window-sash across the room; when the glass is shaken from side to side, there should be no apparent motion of the sash. Ladies often find the protection of a double veil more agreeable than that of any kind of glass, and, in the case of children, the broad brim of a straw hat, lined, if necessary, with dark silk or muslin, may an- swer perfectly well. Weak convex or concave spectacle-lenses may be made of blue or gray glass, but when the lens has a strong curve, a difficulty is met with in the fact that its thick part is necessarily darker than the thin. The sensitiveness of the retina is liable to be ex- hausted by long exposure to the dazzling reflection of sunlight from a wide expanse of snow. This is called snow-bli7idness. Alpine travellers who neg- lect to protect their eyes, sometimes suffer from it seriously. The Esquimaux guard against it by the