A single square array of small lights has a full-silvered mirror in back and a half-silvered mirror in front. A long black box placed in back of the infinity mirror appears to have many rows of lights in it until it is removed!
An interesting sidelight is to use this device to indicate the dynamic range of the eye. Each successive row of lights has an intensity of about 1/2. Approximately twenty rows of lights can be seen by the typical naked eye, so the dynamic range of the eye is at least as great as 2^20, or 1,048,576 to 1.