106 PROCEEDINGS OF THE AMERICAN ACADEMY 



The Recording Secretary read letters from Dr. Herrmann 

 Hagen, acknowledging his election into the Academy, and 

 from Dr. C. A. Martius, of Munich, announcing the death of 

 his father, Dr. Carl Friedrich Philipp von Martius ; also letters 

 relative to exchanges, and a letter from a committee of the 

 Suffolk District Medical Society, asking the co-operation of the 

 Academy in discussing the subject of expert testimony. This 

 communication was referred to a committee of the Academy 

 appointed in March, 1866, to consider the same subject ; and 

 this committee were authorized to add to their number. 



Professor E. C. Pickering made a communication on the 

 Spectroscope, with experimental illustrations of its various 

 constructions and its uses in chemistry and astronomy. 



Six hundred and seventh Meeting. 



April 13, 1869. — Monthly Meeting. 



In the absence of the regular presiding officers, Mr. John 

 A. Lowell was chosen to take the chair. 



Mr. Ritchie exhibited some of the effects of monochromatic 

 light by means of an apparatus producing a bright sodium 

 light. 



Six hundred and eighth Meeting. 



May 11, 1869. — Monthly Meeting. 



In the absence of the regular presiding officers, Hon. C. F. 

 Adams was chosen to take the chair. 



Professor Peirce made the following: communication : — 



*£> 



The phenomena which were ably presented by the distinguished 

 geologist, Mr. Lesley, to the National Academy of Sciences, and which 

 seem to demonstrate that the outer shell of the earth has sensibly 

 shrunk, in some directions at least, since its original formation, naturally 

 invite the attention of physicists to the possible causes of such a 

 result. The most obvious cause of the shrinking of the earth is its 

 cooling. But to shrink two per cent linearly, which is that deduced by 



