Mr. G. G. Stokes on a difficulty in the Theory of Sound, 349 



directed straight towards it. There is no doubt that these 

 changes of tint serve to heighten the illusion of apparent mo- 

 tion when the eye is allowed to wander over the different parts 

 of a complicated pattern. This phenomenon may perhaps be 

 explained by the fact of the sight being most perfect in the 

 axis of vision, or, as Sir David Brewster has expressed it, 

 " the eye has the power of seeing objects with perfect distinct- 

 ness only when it is directed straight upon them, so that all 

 objects seen indirectly are seen indistinctly;" and it may be 

 supposed that impressions received in those parts of the retina 

 used in oblique vision are, as it were, diffused. Thus the red 

 and blue spots, when viewed indirectly, appear tinged with 

 the prevailing colour of the ground of the pattern, — the red 

 spot becoming darker by the influence of the blue round it, 

 and the blue spot lighter by the vicinity of the red ; for it is 

 remarkable that this illusion is not produced with single colours, 

 only with spots of one colour surrounded by a field of the 

 other. 



In concluding these observations, I have only to add, that 

 there cannot be much doubt of the correctness of the view 

 which ascribes the illusory appearance of motion to the change 

 of tint at the edges of the figures. These are matters of fact : 

 but whether the theories offered in explanation of these facts 

 are correct or not, I must leave to more competent observers 

 to determine. 



I have the honour to be, Gentlemen, 



Yours faithfully, 



Henry Taylor. 



LIV. On a difficulty in the Theory of Sound. By G. G. 

 Stokes, M.A., Fellow of Pembroke College, Cambridge*. 



THE theoretical determination of the velocity of sound has 

 recently been the occasion of a discussion between Pro- 

 fessor Challis and the Astronomer Royal. It is not my inten- 

 tion to enter into the controversy, but merely to consider a 

 very remarkable difficulty which Professor Challis has noticed 

 in connexion with a known first integral of the accurate equa- 

 tions of motion for the case of plane waves. 



I would first however observe, that I do not think that we 

 are obliged, in treating the subject to a first approximation, to 

 enter into the consideration of any difficulty which may arise 

 when we come to employ exact equations. In neglecting the 

 squares of small quantities, we adopt a consistent system of 

 approximation, and we arrive at a precise result, namely, the 



* Communicated by the Author. 



