CHAPTER II.

Failure of Sight and the Application of Spectacles 10

Lenses—how to determine their Focal Lengths 12

Brazil Pebbles—Periscopic and Parallel Lenses—Process of 13

Working—Tests of Quality—and Experiment 14

Railroad Spectacles—Neutral Tint Shades 16

Country-made Spectacles—Trial Box of Sight 18