Mr. Norman Campbell on Delta Rays. 285 



recorded , the same feature has been noted. Cases are observed 

 in which the emergence radiation is nearly equal to the 

 incidence, but none in which it is certainly larger. The 

 exact contrary is, o£ course, true of all other forms of 

 radiation. 



10. The quality of the emergence and incidence radiations 

 next requires attention. If the quality of both is the same, 

 the numbers in the columns marked -f- should be the same 

 as those in the corresponding columns marked — . It will 

 be seen that there are considerable divergencies between the 

 + and — columns, and that these divergencies are certainly 

 greater than can be accounted for by mere errors of obser-. 

 vation ; the observations were sufficiently accurate to leave 

 no uncertainty about the second figure, except in the case of 

 column 9. But it will be seen also that the sign of the 

 difference between the + and — columns is not constant in 

 different series, and that in the last column marked "mean," 

 which gives the arithmetic mean of the corresponding numbers 

 for columns 1-8, the divergence has practically disappeared 

 except for the value V=-J. The divergencies are easily 

 explained without assuming any difference in quality between 

 the emergence and incidence radiation. 



It is obvious that the form of the two branches of the 

 curve of fig. 1 will be modified considerably if any point 

 other than 0' is taken as the zero from which all measure- 

 ments are made. 0' has been actually taken as the point 

 corresponding to the condition that no difference of potential 

 is imposed externally on A and B. But it does not follow 

 that this condition represents the state in which A and B are 

 actually at the same potential, still less the state in which 

 there is no resultant electric field in the space between them. 

 In the first place, there may be small thermoelectric dif- 

 ferences of potential in the circuit connecting A and B, and. 

 in the second place, it must be remembered that the sur- 

 rounding cylinder D has a potential widely different from 

 that of A and B. If A and B are not perfectly symmetrically 

 situated with respect to D, there will be a resultant electric 

 field tending to drive the rays to one plate rather than the 

 other. As a matter of fact, it was found that the measure- 

 ments for very small values of Y were much less constant 

 than those for large values, when these sources of error 

 would be less important. The same value could not always 

 be recovered during the same series of observations, and the 

 value varied considerably between different series separated 

 by an interval in which the apparatus had been taken down. 

 In all cases, a much better agreement between the values for 



