Jacksons cylinder is used for-

Ans. is 'c' i.e., Refining cylindrical power and axisJackson's cross cylinder test* Crosscylinder is used to verify the strength and axis of the cylinder prescribed.* The cross cylinder is a mixed combination of various strengths in which spherical component is one half the power of the cylindrical and of opposite sign to with the axis at right angles. The most convenient form is a combination of a -0.5 D sphere with a+1.0 D cylinder which has an effective strength of + 0.5 D cylinder in one meridian and - 0.5 D cylinder in the meridian perpendicular to the first. The cross-cylinder is identified by the strength of its cylinderical component. So, above described cross cylinder is referred as +- 0.5 crosscylinder.* Thus, cross cylinder has following feature : -a) It has one spherical and one cylinderical lens.b) This combination creates two cylinders of equal strength but with opposite sign placed wit their axes at right angle to each other.