Tutor. It is; and therefore people who are very short-sighted while young, will probably see well when they grow old.

James. That is an advantage denied to common eyes.

Tutor. But people blessed with common sight, should be thankful for the benefit they derived while young.

Charles. And I am sure we cannot too highly estimate the science of optics, that has afforded such assistance to defective eyes, which, in many circumstances of life, would be useless without them.

CONVERSATION XVIII.

Of the Rainbow.

TUTOR. You have frequently seen a rainbow?

Charles. Oh, yes, and very often there are two at the same time, one above the other; the lower one is by far the most brilliant.

Tutor. This is, perhaps, the most beautiful meteor in nature; it never makes its appearance but when a spectator is situated between the sun and the shower. It is thus described by Thomson:

-Reflected from yon eastern cloud, Bestriding earth, the grand ethereal bow immense; and every hue unfolds,

Shoots up
In fair proportion, running from the red
To where the vi'let fades into the sky.
Here, awful Newton, the dissolving clouds
Form, fronting on the sun, thy show'ry prism;
And to the sage-instructed eye unfold
The various twine of light, by thee disclos'd
From the white mingling maze.

James. Is a rainbow occasioned by the falling drops of rain?

Tutor. Yes, it depends on the reflection and refraction of the rays of the sun by the falling drops.

Charles. I know now how the rays of the sun are refracted by water, but are they reflected by it also?

Tutor. Yes; water, like glass, reflects some rays, while it transmits or refracts others, You know the beauty of the rainbow consists in its colours.

James. Yes, "the colours of the rainbow" is a very common expression; I have been told there are seven of them, but it is seldom that so many can be clearly distinguished.

Tutor. Perhaps that is owing to your want of patience; I will show you the colours first by means of the prism. If a ray of light s (Plate v. Fig. 31.) be admitted into a darkened room, through a small hole in the shutter x y, its natural course is along the line to d; but if a glass prism a c be introduced, the whole ray will be bent upwards, and if it be taken on any white surface as M N, it will form an oblong image P T, the breadth of which is equal to the diameter of the hole in the shutter.

Charles. This oblong is of different colours in different parts.

Tutor. These are the colours of the rainbow, which are described by Dr. Darwin as untwisted:

Next with illumin'd hands through prisms bright,
Pleas'd they untwist the sevenfold threads of light;

Or, bent in pencils by the lens, convey

To one bright point the silver hairs of day.

James. But how is the light which is admitted by a circular hole in the window spread out into an oblong?

Tutor. If the ray were of one substance, it would be equally bent upwards, and make only a small circular image. Since, therefore, the image or picture is oblong, it is inferred that it is formed of rays differently refrangible, some of which are turned more out of the way, or more upwards than others; those which go to the upper part of the spectrum being most refrangible, those which go to the lowest part are the least refrangible, the intermediate ones possess more or less refrangibility, according as they are painted on the spectrum. Do you see the seven colours?

Charles. Yes, here is the violet, indigo, blue, green, yellow, orange, and red.

Tutor.

These colours will be still more beautiful if a convex lens be interposed, at