Replacement of intraocular contents with gas or silicone oil changes the optical characteristics of the eye. This affects the surgeon's view of the fundus and the patient's vision. Gullstrand's schematic eye was used to calculate the refractive properties of eyes with air or silicone oil in the vitreous cavity or anterior chamber in phakia, aphakia, and pseudophakia. The article gives a general solution of the optical characteristics of the eye with any substance filling the eye or the vitreous cavity. Criteria are shown for selection of appropriate corneal contact lenses for visualization of the fundus during and after vitreous surgery.