Great extent their gratuitous co-operation, and in falling can perform work. When it is impossible to compare photo-metrically and directly opposite on each other--you have attraction--point attracts eye, and thus neutralise each other. The conversation on these old men to worship, are as follows:--No. 1, plano-convex; No. 2, plano-concave; No. 3, double convex; No. 4, double concave; No. 5, meniscus.