192 Retardation of a Particles by Metals. 



emergent or residue range o£ about 3 cm. It will be 

 noticed that our values are somewhat the greater. Our 

 values are also in excess of those given by Taylor*. Further, 

 the variations of air-equivalent for the foils given by both 

 Bragg and Taylor are somewhat more pronounced over the 

 range they examined than are our values. The difference 

 is probably due to the two methods of measurement. It 

 may be convenient here to give a few points comparing the 

 two methods of determining air-equivalents, — ionization and 

 scintillation. In the case of the scintillation method no 

 trouble arises from scattering, as is possible in the ionization 

 method. In the scintillation method a much smaller area 

 of foil, generally only about 2 sq. mm., is used than in the 

 ionization method, where an area ordinarily not less than 

 0'5 sq. cm. is employed. In the case of foils which are in 

 any way ununiform, a better average is therefore obtained 

 in the ionization method. This can be remedied to some 

 extent by determining the equivalence in several places. 

 However, difficulty arises in the scintillation method when 

 the foils under investigation contain small holes or local 

 irregularities, for in that case there is a tendency to measure 

 the thinnest portion of the foil. These small local irregu- 

 larities were found in all the foils examined except gold. 

 In our experiments we avoided the error to some" extent by 

 using composite sheets of many thicknesses of foils. Any 

 serious irregularities can be instantly noticed by the fact 

 that they allow a particles of different velocities to fall on 

 the screen, causing sciiitillatTons" of 'different brightness. 



The scintillation method, as used above, is convenient in 

 that it does not involve any extensive apparatus. The 

 observations are more direct and can be made much quicker 

 than by the ionization method. A source such as we have 

 used is not necessary, for instance a moderately strong 

 preparation of polonium serves very well. 



A theoretical investigation of the above results on the 

 variation of air-equivalent is somewhat difficult in the 

 absence of definite information of the change of velocity of 

 the u particles in passing through the various foils. The 

 velocity is obviously more fundamental than the range, 

 which is of course measured in an arbitrary substance. 

 It appears, therefore, preferable to postpone further dis- 

 cussion until the completion of some experiments on velocity 

 curves in various substances now in progress by Dr. Taylor 

 and one of us. 



* Luc. cit. 



