51 
at the total solar Eclipse in 1882. 
axis y of rectangular coordinates with the upper extremity of the 
focal line, A, as origin : ordinates measured in the direction AB 
Fig. 6. Geometrical Construction of Reflector, when the position of a point 
on one of the mirrors, and the position and length of the focal line, are given. 
are to be reckoned positive, those measured in the reverse direction 
are to be reckoned negative. 
4—2 
