What will be the curvature correction for a staff reading in levelling for a distance of 1000 m?

Seven consecutive readings taken with a levelling instrument are 1.345, 1.525, 0.785, 1.105, 1.855, 2.005, and 1.985. If the instrument was shifted after the third and fifth readings, the readings recorded in the Back Sight (BS) column would be:

In levelling, the line of collimation is known as ___________.