Hello Mrs. Jones, this instrument is called an autorefractor/keratometer and Dr. Lee thinks it is practically magic. It works by bouncing a beam of light off the back of your eye, as that light is reflected back through the optical components of your eye, it transfers the aberrations in your optical system to a computer which calculates an objective idea of your prescription. Dr. Lee uses this in assessing your final prescription. It uses the reflection of another light source off the front of your eye to measure the shape. It also allows me to assess the continuity of the reflection, providing us with information regarding the quality of your tear film. In many patients, a reduction in the quality of the tear film interferes with clear, comfortable vision.