ABSTRACT
A num ber of tubes were cut in graduated lengths of 9-6 cm., 12*3 cm., 17-5 cm., 20*6 cm., and a corresponding num ber of cork stops, giving (with the wall of the tube) lenticular apertures of 27 mm. x 7 m m., 23 x 11, 24 x 13*5, and 26 x 18*5 respectively. See Fig. 164.
