344 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



second plate B h, exactly similar to A a, is laid on the anterior surface 

 of the glass block G. The whole is mounted in a brass box, for 

 convenient manipulation, and for limiting a given portion f^ of the 

 anterior porcelain plate A, exactly equal in size to the sectional 



Fig. 83. 



P, 



Fig. 85. 



surface /i of the glass block. The plates A and B are ground down 

 to such a thinness that they are just non-transparent for direct rays, 

 but give a strong diffused illumination, radiating very uniformly in 

 all directions. 



If now the apparatus is directed to the sky, or held close to a 

 gas-flame (the plate B being turned towards the observer and shaded 

 from any incident light*), 07ie of the fields .^i or j/^, opjiosite to/, or/^, 

 will appear decidedly and considerably brighter than the other. 



At the March meeting this fact was at once triumphantly 

 accounted for by an angular aperturist. " The light passing through 

 " the air space is unimpeded and unabsorbed, and therefore illuminates 

 " that part of the plate B which is opposite to it more strongly. In 

 " passing through the glass block, however, it is absorbed, and so 

 " shows a feebler illumination." On removing the plate B, however, 

 it was seen that it was the field ^^ opposite to f.2, and with the air- 

 space intervening, that had the feebler illumination, as if the light 

 had passed through a fog; whilst /i, where the light had passed 

 through the glass block, gave the greater illumination ! 



What does this experiment show ? As the two fields, /, and f^ 

 * 111 Fig. 85 both of the plates A and B are supposed to be removed. 



