[Illustration: Here the ray AB received on a prism, would have the blue part bent up to C; the yellow part to D; and the red part no further than E.] Q. _What is meant by the REFRACTION of a ray?_ A. _Bending it_ from its straight line. Thus the ray AB of the last figure is refracted at B into three courses, C, D, and E. Q. _What is the cause of a RAINBOW?_ A. When the clouds opposite the sun _are very dark_, and rain is _still falling_ from them, the rays of the bright sun _are divided by the rain-drops_, as they would be by a prism. [Illustration: Let A, B, and C be three drops of rain; SA, SB, and SC three rays of the sun. SA is divided into the 3 colours; the blue and yellow are bent _above_ the eye D, and the _red_ enters it. The ray SB is divided into the three colours; the blue is bent _above_ the eye, and the red falls _below_ the eye D; but the _yellow_ enters it. The ray SC is also divided into the three colours. The blue (which is bent most) enters the eye; and the other two fall below it. Thus the eye sees the blue of C, and all drops in the position of C; the yellow of B, and of all drops in the position of B; and the red of A, &c.; and thus it sees a rainbow.] Q. _Does EVERY person see the SAME colours from the SAME DROPS?_ A. No; _no two persons_ see the _same rainbow_. To another spectator the rays from SB might be _red_ instead of yellow; the ray from SC, yellow; and the blue might be reflected from some drop below C. To a _third_ person the red may issue from a drop above A, and then A would reflect the yellow, and B the blue, and so on. Q. _Why are there often TWO RAINBOWS at one and the same time?_ A. In _one_ rainbow we see the rays of the sun _entering the rain-drops at the top_, and reflected to the eye _from the bottom_. In the _other_ rainbow, we see the rays of the sun _entering the rain-drops at the bottom_, and reflected to the top, whence they reach the eye. [Illustration: Here the ray SA strikes the drop at A,--is refracted or bent to B,--is then reflected to C, where it is refracted again, and reaches the eye of the spectator.] [Illustration: Here the ray SB strikes the drop at B,--is refracted to A,--is then reflected to C,--is again reflected to D, when it is again refracted or bent till it reaches the eye of the spectator.] Q. _Why are the COLOURS of the SECOND bow all REVERSED?_ A. Because in _one_ bow we see the rays which enter at the _top_ of the raindrops, _refracted from the bottom_: But in the _other_ bow we see the rays which enter at the _bottom_ of the raindrops (after two reflections), _refracted from the top_. [Illustration: Here A, B, C, represent three drops of rain in the PRIMARY (or inner) RAINBOW. The _least_ refracted line is RED, and BLUE the _most_.