/ 
C. A. Young on a method of determining the Level-error. 349 
eye-piece N, by which light can be thrown upon the dot through 
the tube of the microscope. This enables us to measure the 
flection in the manner to be descri c 
Opposite O is fixed a prism shaped like that of a camera 
lucida, which by two total reflections bends the light through a 
vertical distance between the dot and its image formed by re- 
ed. 
vised as to be free from tremors as i 
recently described by J. H. Lane, of the U.S. C 
There is difficulty i ing the apparatus so that the 
: no difficulty in arranging the ap 
ordinary illumination of the wires at the eye-piece of the teles- 
