While experimenting with his mirror prototypes, Silver found he could adjust the lens in the mirrors by varying the amount of silicone oil between two flexible membranes.
When you press the shutter release on the camera, a complex sequence of events occurs, including setting the aperture in the lens, flipping up the mirror of an SLR and a lot more.
Spherical Aberration — causes light rays passing through a lens (or reflected from a mirror) at different distances from the optical center to come to focus at different points on the axis.