PHILOSOPHICAL SOCIETY OP WASHINGTON. 25 



This level surface is that in which civil engineers are interested, 

 but not that which the higher geodesy seeks to determine, nor 

 that which astronomers would use if, as in the works iuvolving 

 parallax, the exact distance of a station were required from the 

 assumed centre of the earth. 



If we would determine the altitudes above the adopted normal 

 ellipsoid, our levellings require indeed a correction for " local 

 attraction of the plumb line," which, as Baeyer has shown 

 for Germany (Astron. Nach., No. 1993), may amount to many 

 feet in the case of our western plateaus. 



The President in this connection gave some historical reminis- 

 cences of early surveys for the Erie canal. 



'79th Meeting. January 2, 1875. 



Yice-President Taylor in the Chair. 

 Eight members present. 



Mr. J. J. Woodward made remarks 



ON THE MODERN MICROSCOPE, NOBERT'S LINES, AND THE ATTEMPTS 

 OF OTHERS TO CONSTRUCT THEM ; 



followed by a conversational discussion extending to the theory 

 of vision and the structure of the human retina, in which Messrs. 

 Taylor, Skinner, and Woodward participated. 



Mr. E. B. Elliott made remarks 



on. the transition in GERMANY, AND THE SCANDINAVIAN NA- 

 TIONS OP SWEDEN, NORWAY, AND DENMARK, PROM THE SILVER 

 STANDARD OP COINAGE AND MONEY OP ACCOUNT TO A GOLD 

 STANDARD. 



Adjourned Meeting. January 9, 1875. 



The President in the Chair 



Twenty-two members and visitors present 



