What is the term for a lens consisting of two cylinders of equal but opposite powers?

The term for a lens that consists of two cylinders of equal but opposite powers is a Cross Cylinder. This type of lens is often used in corrective optics, particularly for astigmatism, where it can help correct vision by compensating for the differences in curvature of the eye's cornea.

A Cross Cylinder effectively combines two cylindrical lenses oriented at right angles to each other, and the equal but opposite power means that one cylinder compensates for the aberration caused by the other. This allows for precise adjustments in vision correction, matching the specific refractive error of the eye.

In contrast, a spherical lens has a uniform curvature and is not suitable for correcting astigmatism. A cylindrical lens, while it can correct for one meridian of astigmatism, does not have the opposing power that a Cross Cylinder has. A compound lens generally refers to a lens made up of two or more simple lenses combined together but does not specifically refer to the use of cylindrical powers in this context.