

626 Mr. Norman Campbell on Relativity 



The totals at the foot of the table have no significance 

 except that they serve to show by their agreement that the 

 differences between the calculated and observed values in 

 individual experiments lie pretty evenly on either side. The 

 measurements entirely confirm the theory. 



Summary. — General considerations lead to the conclusion 

 that the unsymmetrical character of the distribution of 

 illumination in diffraction-patterns observed at oblique 

 incidences, must be an obliquity effect. This is entirely 

 verified by photometric work on the diffraction-pattern. 

 The measurements show that the obliquity-factor follows the 

 cosine-law, which may be stated thus : — " In the hemi- 

 spherical wavelets emitted by each element of a transmitting 

 aperture or reflecting surface upon which waves are incident 

 at any angle, the amplitude of the light vector is, at any 

 point in the plane of incidence, proportionate to the cosine 

 of the angle made by the line joining that point and the 

 element, with the normal to the plane of the element." 



This investigation was commenced at the Physical Labo- 

 ratory of the Indian Association for the Cultivation of 

 Science, Calcutta. My removal from that station necessitated 

 its being completed elsewhere. 



LXXII. Relativity and the Conservation of Momentum. 

 By Norman Campbell *. 



1. TN the March number of the ' Philosophical Magazine ' 

 JL Dr. Tolman attempts to show that the number of 

 independent propositions necessary to formulate the theory 

 of electromagnetic action may be reduced if the " law " of: 

 the conservation of momentum be assumed. This conclusion 

 appears to me fallacious, and the fallacy to be one which 

 deserves notice. The error is introduced, in my opinion, by 

 a false application and by a misunderstanding of a result 

 proved by Prof. Lewis and Dr. Tolman in a previous paper 

 (Phil. Mag. [6] xviii.p. 510, 1909). This result consisted in a 

 demonstration that the formula for the variation of the mass 

 of a body with its velocity relative to the observer could be 

 deduced by assuming only the " law " of the conservation of 

 momentum and the fundamental principles of relativity. It 

 will be convenient to examine this conclusion first. 



2. The argument as it is stated appears to me to contain 

 many of the errors and confusions noted in a recent paperf . 



* Communicated by the Author. 



t " The Common Sense of Relativity," Phil. Mag. April 1911. 



