horizontal line

line parallel to the mean spheriodal surface of earth

line passing through the center of cross hairs and the center of eye piece

line passing through the objective lens and the eye-piece of a dumpy or tilting level

foresight only

backsight only

foresight and backsight

foresight and intermediate sight

backsight

intermediate sight

foresight

all of these

99.345 m

100.345 m

100.655m

101.870m

concave

convex

plano-convex

plano-concave