i went to an opthamologist today for an eye checkup.. and his assistant told me to put my chin on some machine and that machine sort of showed me an image of some scenery sort of thing. Now, the machine started to do something.. and i could see the image getting blurred again and becoming sharp again. It finally stopped at some point where the image became sharp. Then it did the same thing for the other eye. The assistant then noted down some numbers from that machine. what exactly does that machine do, what's it's name.. and how does it know when my eyes see a perfectly sharp image?