140 
POPULAR SCIENCE REVIEW. 
The lenses should be of the best crown glass, free from all 
spots, flaws, streaks, or air-bubbles, and perfectly colourless. 
Improvements of manufacture have made this glass far better 
than Brazilian pebbles, once greatly in repute, but which, 
unless cut exactly at right angles to the axis of the original 
crystal, are apt to be very disturbing to vision. 
The frames should be of blue steel, light, strong, and per- 
fectly fitted to the wearer. The centre of each ring should be 
exactly opposite the pupil of its corresponding eye; and the 
nose-piece of such a height and curvature as to place the lenses 
in their right position. Concave spectacles should be worn as 
close to the eyes as possible, convex spectacles a little farther 
away. When the latter are used on account of presbyopia 
only, they may even be half way down the nose ; and it is 
convenient to have the rings flattened on the top, so that the 
, wearer may look through them at his book, and over them at 
distant objects. Finally, spectacles should be kept perfectly 
clean, never thrown about carelessly, nor pocketed without 
their case. The lenses should be wiped, when necessary, with 
a piece of clean soft wash-leather ; and replaced if scratched or 
clouded. They are often essential auxiliaries to vision; and 
they should be cared for like the eyes themselves. 
