It is neither the first nor the last picture to go viral on the Internet because of the difference in perception of different users when looking at it. But it might be the most useful.Are you sure that your eyesight is perfect? Do you know that you're a bit of nearsighted or farsighted but suspect you might have something else? Well, this photo appears to be able to solve that mystery in a couple of seconds.What number do you see?The system is very simple. There seems to be a hidden number among the points that make up this photograph, but (that's the mystery): not all users see the same number.The number that you see reveals if you have hyperopia (poor vision up close), astigmatism (blurred vision), myopia (poor vision from afar) or a mixture of these.Do you dare to test your eyesight? Well, hit the play button and find out.Disclaimer: it's just a picture. If you think you might suffer from a serious problem with your eyesight, always consult a specialist.