on Mr. Potter's Experiment on Interference. 167 



changes in the rule for forming the versed sine from the co- 

 sine, and in fact follow exactly the same law. 



In explaining to a young student in mechanics the motion 

 of elastic balls when one has impinged on another which was 

 at rest, we might perhaps make two separate cases distin- 

 guished by the circumstances of the impinging ball being the 

 greater, or the impinging ball being the smaller; and we 

 should point out, that in the former case the motion of the 

 impinging ball after impact was in the same direction as be- 

 fore; while in the latter case the motion after impact was in 

 the direction opposite to its first motion. A more advanced 

 student would perceive that both were included in the general 



A — B 

 formula - A — « *>• Thus it is with the reflexion of light from 

 A-f B to 



the inner or outer surface of glass : the mechanical conditions 

 appear to be precisely similar to those which I have mentioned, 

 and the theoretical result is similar; namely, that whereas in 

 one case we are bound to suppose the remaining motion 

 (which produces the reflected ray) to retain the same direc- 

 tion as before, in the other case we are equally bound to sup- 

 pose that the remaining motion has a direction opposite to 

 that which it had before. I lay smaller stress upon this part 

 of the theory than upon the other, because I consider the me- 

 chanical part of the theory of undulations generally as less 

 complete than the geometrical part : but what I have stated 

 shows clearly that there is nothing arbitrary in this change of 

 sign ; but that it , is absolutely required by theory as far as 

 theory goes. I may add, that in making a complete mathe- 

 matical investigation in any part of the theory of undulations, 

 — for the explanation, for instance, of the most complicated 

 phenomena of polarization, — the "demand of half an undu- 

 lation, " which has made so strong an impression on Mr. 

 Potter, never occurs, 



I rejoice that Mr. Potter has seriously undertaken to com- 

 pare his experiments with the mathematical results derived 

 from the theory of undulations. I hope (for the sake of the 

 science) that he will continue his experiments; and I hope 

 (more particularly for his own conviction) that he will con- 

 tinue the corresponding mathematical investigations. If the 

 comparison between them be continued in the same philoso- 

 phical spirit as that which marks his last paper, I can with 

 confidence predict one result : — Mr. Potter will very soon be- 

 come an undulationist. 



I am, Gentlemen, your obedient Servant, 



Observatory, Cambridge, Feb. 2, 1833. G. B. AlRY. 



