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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 



a 

 b 



Fig. 2. 



Fig. 1. 



Fig. I. Diagram of lead cell used to hold the radium. The upper portion of the 

 flgiu-e is a vertical section; the lower a horizontal section as the level h. Magnified 

 one and one third diameters. 



a. Paper screen, two thicknesses of black paper such as is used to protect photo- 

 graphic plates; 6, 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; b, Radium; c. Glass floor of cell; d, sealing-wax rim. 



