The day after Christmas was also the traditional day on which the aristocracy distributed presents (boxes) to servants and employees  a sort of institutionalized Christmas-bonus party. The servants returned home, opened their boxes and had a second Christmas on what became known as Boxing Day.
300 mm. For a 35 mm camera with a diagonal of 43 mm, the most commonly used normal lens is 50 mm, but focal lengths between about 40 and 58 mm are also considered normal. The 50 mm focal length was chosen by Oskar Barnack, the creator of the Leica camera.
Similar to the design in Figure 5, each segment of the waveguide only creates a small field of view, and the multiple segments together create a full field of view of 24 degrees by 16 degrees in the horizontal and vertical directions, respectively. The equivalent focal length of the viewing optics is 100mm.