348 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
wise than those arising from differences in their penetrating power. 
An explanation of the lack of success in determining their differences 
of biological action is to be found in the difficulties encountered in 
isolating and measuring the radiations. Furthermore if a pure radi- 
ation of known intensity be obtained, the changes as to intensity 
and quality of all three types of radiation in their passage through 
the tissue remains to be reckoned with. 
Drosophila. 
In the following experiments with the eggs of Drosophila the quality 
and intensity of the alpha, beta, and gamma radiations are known 
only approximately; but the complication due to the change of char- 
acter of the radiations at different depths from the exposed surface 
of the egg is largely eliminated by the minuteness of the egg (less than 
0.1 mm. in diameter) and the thinness of the egg case. 
In most exposures a hundred milligrams of impure radium of one 
Fig. 2. Lanes ile 
Fig. 1. Diagram of lead cell used to hold the radium. The upper portion of the 
figure is a vertical section; the lower a horizontal section as the level b. Magnified 
one and one third diameters. 
a. Paper screen, two thicknesses of black paper such as is used to protect photo- 
graphic plates; b, radium: c, lead cell. 
Fig. 2. Diagram showing construction of the cell used to contain the radium (the 
weaker sample) in most of the experiments except those on the eggs of Drosophila. 
Magnified one and one half diameters. 
a. Mica roof of cell; 6, Radium; c, Glass floor of cell; d, sealing-wax rim. 
