INVESTIGATION OF THE CURVATURE OF SURFACES. 48 
and this is the expression of the condition that the lines whose direction cosines are 
proportional to ZL, M N; L+dL,M+dM,N+dQN,dzu,dy,dz, namely two conse- 
cutive normals and the are between them on the surface, should lie in the same plane. 
Hence we see that substituting in equations of (15) we can get the equations of the 
lines of curvature in the shape 


LASER CREER CAMP AIS 
b—k, l, 70e EN | ea Mm, JS) | ee n, L 34) 
DR CEE en: PNR Dr i bho | AC 
M, N, Ny, D, Te M, 
