s in the colony. We have PIZ +vvz). Ra+P( )wz+! Ribv2z2 +O(w3) r,-.R(Z+wz)= R(21+ ralwz +I R. (Znv2z2 +O(tv3) (11) We have made the substitutions p,-•P(Z+ wz) and rz -.R(Z+wZ). and (11) gives the Taylor expansions of these quantities in terms of their first and second derivatives at intensity Z. (For