it consists of two equal piano convex lenses

the curved surfaces of plano-convex lenses face each other

the two lenses are separated by a distance equal to 2/3 of the focal length of either lens.

All of these

bridge carrying railway below road

bridge carrying road below railway

bridge carrying road and railway at the same level

a level crossing.

embankment on the centre line

excavation on the centre line

earth work on the centre line

none of these.

positive compensating error

negative compensating error

positive cumulative error

negative cumulative error.

linear measurements only

angular measurements only

both linear and angular measurements

none of these.