“…A large portion of the work on this subject has been purely observational in nature and involves but little experimental data (exceptions are Huskins and Wilson (1938), Matsuura (1937), and Swanson (1942». Nebel's (1941 recent review is illustrative of the many difficulties encountered in this field through deceptive imagery, but it is evident that additional observations, obtained under varying environmental conditions, are needed to supplement the data already at hand.…”