276 Mr. Potter's Reply to Professors Airy and Hamilton. 



800 tuq (hum b e^iu.riJana'iJLg toqoTq 



— - = 1*0191 = diffusion- volume of defiant gas, by ex- 

 785 



penment. 



The contraction in this experiment is a little above the 

 theoretical quantity. In another experiment with different 

 gas, the contraction was even greater, indicating a diffusion- 

 volume = 1'0303; but the presence of a minute quantity of 

 carburetted hydrogen, or some lighter hydro- carburet, was 

 suspected, from the rapidity of the contraction in this case. 

 [To be continued.] 



XLV. A Reply to the Remarks of Professors Airy and Ha- 

 milton on the Paper upon the Interference of Light after 

 passing through a Prism of Glass. By R. Potter, Juti., Esq* 



~]\/I Y paper in the February Number of this Journal, On the 

 •** Interference of Light which has been refracted by a Prism, 

 having been noticed in the last Number by Professors Air}^ 

 and Hamilton, I hasten to reply to their remarks ; although I 

 am very far still from being prepared to enter completely into 

 the subject of the velocity of light in traversing refracting 

 media. I must accordingly still refer solely to the experiment 

 with the prism, leaving to another time the appeal to a more 

 direct and less intricate experiment; with which I have been 

 long occupied, but which I have not yet been able to get 

 through, on account of its requiring apparatus which 1 did not 

 before possess, and which I find still requires further additions 

 to produce some minute adjustments. 



The papers of the two learned Professors have tended much 

 to strengthen my previous expectation as to what will be found 

 eventually to be the real velocity of light in passing through 

 refracting bodies ; for both of them have raised objections to 

 my conclusions only upon points on which I had myself in 

 the outset some misgivings. 



Upon that which is proposed by Professor Airy, I was at 

 great pains to satisfy myself experimentally. In using com- 

 mon light, or light considerably heterogeneous, and when the 

 distance between the images of the luminous point is too small 

 in comparison with the distance of the prism from them, the 

 bands or interference-fringes might be supposed, from what 

 we observe, merely to dilate. This appears to be the way in 

 which Professor Airy has tried it, which I conceive he has 

 done ; and I placed the original paper in Mr. Coddington's 

 hands at Oxford. 



To satisfy myself upon this point, I used the red light given 

 by the solution of iodine in hydriodic acid, which, when of 



* Communicated by the Author. 



