The Most convenient form of cross cylinder is a combination of -
Jackson's 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: Jackson's cross cylinder Reference : A K KHURANA OPHTALMOLOGY,Edition 4,Page-555
-0.5 Diopter sphere with +1 Diopter Cylinder
+0.5 Diopter sphere with -1 Diopter Cylinder
+0.25 Diopter sphere with -0.5 Diopter Cylinder
-0.25 Diopter sphere with +0.5 Diopter Cylinder
Optics and refraction