Tests to determine adequacy of windows and lenses.

Impact tests. A 4-pound cylindrical weight with a one-inch diameter hemispherical striking surface will be dropped (free fall) to strike the window or lens in its mounting or the equivalent thereof at or near the center. At least three out of four samples shall withstand the impact according to the following table:

              +----------------------------------+----------+
              |                                  |   Height |
              |  Overall lens diameter (inches)  |   of fall|
              |                                  |  (inches)|
              +----------------------------------+----------+
              | Less than 4......................|      6   |
              |                                  |          |
              | 4 to 5...........................|      9   |
              |                                  |          |
              | 5 to 6...........................|     15   |
              |                                  |          |
              | Greater than 6...................|     24   |
              +----------------------------------+----------+