<prompt> The Most convenient form of cross cylinder is a combination of --0.5 Diopter sphere with 1 Diopter Cylinder0.5 Diopter sphere with -1 Diopter Cylinder0.25 Diopter sphere with -0.5 Diopter Cylinder-0.25 Diopter sphere with 0.5 Diopter Cylinder  <response>: Jacksons cross cylinder test: It is used to verify the strength and axis of the cylinder prescribed. The crosscylinder is a combination of two cylinders of equal strength but with opposite sign placed with their axes at right angles to each other and mounted in a handle . The commonly used crosscylinders are of -0.25 D and -0.5 D Image: Jacksons cross cylinder Reference : A K KHURANA OPHTALMOLOGY,Edition 4,Page-555 