It appears from the things explained above that the progression or propagation of a small part of a wave of light is properly what one calls a ray.

Now these rays, instead of being straight as they are in homogeneous media, ought to be curved in an atmosphere of unequal penetrability.

For they necessarily follow from the object to the eye the line which intersects at right angles all the progressions of the waves, as in the first figure the line AEB does, as will be shown hereafter; and it is this line which determines what interposed bodies would or would not hinder us from seeing the object.

For although the point of the steeple A appears raised to D, it would yet not appear to the eye B if the tower H was between the two, because it crosses the curve AEB.

But the tower E, which is beneath this curve, does not hinder the point A from being seen.