The best material for the construction of globes is that which
possesses the maximum of translucency coupled with non-transparency,
_i.e._, a material which passes the highest proportion of the light
falling upon it, and yet disperses that light in such different
directions that the glowing body cannot be seen through the globe. Very
roughly speaking, plain white glass, such as that of which the chimneys
of oil-lamps and incandescent gas-burners are composed, is quite
transparent, and therefore affords no protection to the eyesight; a
protective globe should be rather of ground or opal glass, or of plain
glass to which a dispersive effect has been given by forming small prisms
on its inner or outer surface, or both. Such opal, ground, or dispersive
shades waste much light in terms of illuminating power, but waste
comparatively little in illuminating effect well designed, they may
actually increase the illuminating effect in certain positions; a tinted
globe, even if quite plain in figure, wastes both illuminating power and
effect, and is only to be tolerated for so-believed aesthetic reasons.
Naturally no globe must be of such figure, or so narrow at either
orifice, as to distort the shape of the unshaded acetylene flame--it is
hardly necessary to say this now, but some years ago coal-gas globes were
constructed with an apparent total disregard of this fundamental point.
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