But those who wish to assure themselves through experience and in concrete fashion of what I have just said, that the rays farther from the axis such as Ag and Am, after their refraction intersect the axis at A closer to than the others, have only to completely cover a rather large lens with paper in which they have made with a pin four holes arranged in a straight line, two of which are some equal distance from the axis and correspond to the edges of the lens, and the two others rather near the axis.