ADDENDUM. 



On page 120, line 2, add the following paragraph : 

 The phenonema seem to be independent of the material of which 

 the jaws of the slit are composed, for the same results followed in 

 the case of brass, steel, and obsidian. In the case of the slit made 

 of obsidian, the effects were a little more marked than with the 

 metals, which may be attributed to the greater accuracy of the 

 edges. 



148 



