272 PROCEEDINGS OP THE AMERICAN ACADEMY 



The theoretical values in the above tables were obtained by a 

 grajiliical solution, as the formulEe, after undergoing the changes 

 necessarily made, because of the sound waves being of considerable 

 magnitude, became quite cumbersome, and extreme accuracy was not 

 required. The value of ). was carefully determined and properly 

 corrected for temi^erature. 



It should be remarked that all of the above experiments were made 

 in the large hall of the Institute, a room 92' by 65'. 



The phenomena of the diffraction of sound are not so distinctly 

 marked as those of the diffraction of light. An examination of the 

 tables, which are the results of a most careful series of observations, 

 show that we are not warranted in accepting them as a basis for such 

 excellent further work as has been done in the case of light. It is 

 quite possible, of course, to calculate, from the positions of the fringes, 

 the values of X and therefore of V ; to determine the temperature of the 

 room in which the experiments are carried on ; or, given these quan- 

 tities, to deduce the values of physical quantities which are intimately 

 connected with the propagation of sound, and to determine acoustic 

 quantities analogous to those similarly deduced in physical optics : but 

 the method is difficult, uncertain, requires the use of a large hall, 

 physical annoyance to the observer, and, above all, is not susceptible of 

 the desired degree of accuracy. 



Tlieir chief value seems to lie in their reactive effect on physical 

 optics. In acoustics we are sensible/ aware that we are dealing with 

 waves propagated in an elastic medium. These waves may be felt 

 and even seen.* Finding similar effects in optics to those here 

 observed, we immediately refer these similar effects to similar causes, 

 and so place our explanations of the diffraction of light and of the 

 various cases of ethereal interference on a much firmer basis. The 

 experiments show, too, that the principles of Fresnel and Huyghens, 

 announced for the ether, are also applicable to our atmosphere. 



* Expts. of Topfer. Fogg. Ann. 1867. 



