It is found in Aldis's “Tract on Double Refraction,” p. 7, in Preston's “Theory of Light,” third edition, p. 328, and in Basset's “Treatise on Physical Optics,” p. 115.

The change in direction of a beam of light as it travels from one material to another is called refraction. The normal is a construction line drawn at right angles to the surface of the glass block.