1903.] the Retroccdcarine Region of the Cortex Cerebri. 65 



the Human brain usually consists of an anterior part, which is an 

 anterior limiting sulcus of the area striata, and a posterior part, which 

 is a mere indentation (or indentations) of part of the mesial area 

 striata, therefore it is not exact to speak of these similarly named 

 furrows as being strictly homologous.] 



" On the Acoustic Shadow of a Sphere." By Lord Eayleigh, 

 O.M., F.ES. With an Appendix, giving the Values of 

 Legendre's Functions from P to P 20 at Intervals of Five 

 Degrees. By Professor A. Lodge. Eeceived December 28, 

 1903— Kead January 21, 1904. 



(Abstract.) 



The problem here considered is that of the intensity of sound at the 

 various points of a rigid and fixed sphere on which plane waves 

 impinge, or reciprocally the intensity at a distance^n various directions 

 due to a source of sound situate upon the surface of the sphere. The 

 analytical solution is readily given, bujb in the interpretation everything 

 depends upon the ratio of the wave-length (2-njk) to the circumference 

 (27rc). If kc be small, the sphere has little effect. In my book on the 

 " Theory of Sound," § 328, I have considered (but only-" for certain 

 special directions) the case of kc = 2. The extension to various 

 directions is now given • and the calculation is pushed to the case of 

 kc = 10, about as far as is practicable. For this purpose the values of 

 Legendre's Functions up to P 2 o are required. 





I 



x = 10. 





e. 



4(F 2 + a 2 ). 



0. 



4(F 2 + G). 







3-8300 



105 



1-06117 



15 



3-8176 



120 



0-56815 



30 



3-7148 



135 



0-27890 



45 



3-4978 



150 



0-13338 



60 , 



3-1098 



165 



0-09492 



75 



2-4984 



170 



0-12591 



90 ... , 



1-7510 



175 



0-69395 







180 



1-09263 



The table gives the intensity in directions making angles with 

 the radius which passes through the source. On the same scale the 

 intensity would be unity were the sphere removed. The most 

 interesting feature is the existence of a fairly good shadow between 

 135° and 170°, and the subsequent rise of intensity in the neighbourhood 



VOL LXXIII. F 



