mountain by a spectator in a valley, by which a complete circular rainbow has been exhibited. James. And I once remember standing on Morant's Court Hill, in Kent, when there was a heavy shower, while the sun shone very bright, and all the landscape beneath, to a vast extent, seemed to be painted with the prismatic colours.

Tutor. I recollect this well: and perhaps to some such scene Thomson alludes: it was certainly the most beautiful one I ever beheld :

These, when the clouds distil the rosy shower,
Shine out distinct adown the watery bow:
While o'er our heads the dewy vision bends
Delightful, melting on the fields beneath.
Myriads of mingling dyes from these result,
And myriads still remain; infinite source
Of beauty, ever blushing, ever new.

Charles. You have not explained the principles of the upper or fainter bow.

Tutor. This is formed by two refractions and two reflections: suppose the ray Tr, to be entering the drop в at r. It is refracted at r, reflected at s, reflected again at t, and refracted as it goes out at u, whence it proceeds, being separated, to the spectator at g. Here the colours are reversed; the angle formed by the red ray is 51°, and that formed by violet is 54°.

James. Does the same thing happen with regard to a whole shower, as you have shown with respect to the two drops?

Tutor. Certainly, and by the constant falling of the rain, the image is preserved constant and perfect. Here is the representation of the two bows. (Plate v. Fig. 33.) The rays come in the direction s A, and the spectator stands at E with his back to the sun, or, in other words, he must be between the sun and the shower.

This subject may be shown in another way; if a glass globule filled with water be hung sufficiently high before you, when the sun is behind, to appear red, let it descend gradually, and you will see in the descent all the other six colours follow one another. Artificial rainbows may be made with a common watering pot, but much better with a syringe fixed to an artificial fountain; and I have seen one by spirting up water from the mouth; it is often seen in cascades, in the foaming of the waves of the sea, in fountains, and even in the dew on the grass.

Dr. Langwith has described a rainbow, which he saw lying on the ground, the colours of which were almost as lively as those of the common rainbow. It was extended several hundred yards, and the colours were so strong, that it might have been seen much farther, if it had not been terminated by a bank, and the hedge of a field.

Rainbows have also been produced by the reflection of the sun's beams from a river: and

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Mr. Edwards describes one which must have been formed by the exhalations from the city of London, when the sun had been set twenty minutes.*

CONVERSATION XIX.

Of the Refracting Telescope.

Tutor. We now come to describe the structure of telescopes, of which there are two kinds; viz. the refracting and the reflecting telescope.

Charles. The former or refracting telescope, depends, I suppose, upon lenses for the operation; and the reflecting telescope acts chiefly by means of mirrors.

Tutor. These are the general principles upon which they are formed; and we shall devote this morning to the explanation of the refracting telescope. Here is one completely fitted up.

James. It consists of two tubes, and two glasses.

Tutor. The tubes are intended to hold the

* See Phil. Trans. Vols. VI. and L.

glasses, and to confine the boundary of the view. I will therefore explain the principle by the following figure (Plate v. Fig. 34.) in which is represented the eye A B, the two lenses m n, op, and the object xy. The lens op, which is nearest to the object, is called the objectglass, and that m n nearest to the eye is called the eye-glass.

Charles. Is the object-glass a double convex, and the eye-glass a double concave ?

Tutor. It happens so in this particular instance, but it is not necessary that the eye-glass should be concave; the object-glass must, however, in all cases, be convex.

Charles. I see exactly, from the figure, why the eye-glass is concave: for the convex lens converges the rays too quickly, and the focus by that glass alone would be at E and therefore the concave is put near the eye, to make the rays diverge so much as to throw them to the retina before they come to a focus.

Tutor. But that is not the only reason: by coming to a focus at E, the image is very small, in comparison of what it is when the image is formed on the retina, by means of the concave lens. Can you, James, explain the reason of all the lines which you see in the figure?

James. I think I can:-there are two pencils of rays flowing from the extremities of the arrow, which is the object to be viewed. The rays of the pencil flowing from x, go on di

verging till they reach the convex lens op, when they will be so refracted by passing through the glass, as to converge, and meet in the point x. Now the same may be said of the pencil of rays which comes from y; and, of course, of all the pencils of rays flowing from the object between x and y. So that the image of the arrow would, by the convex lens, be formed at E.

Tutor. And what would happen if there were no other glass?

James. The rays would cross each other and be divergent, so that when they got to the retina, there would be no distinct image formed, but every point as x or y, would be spread over a large space, and the image would be confused. To prevent this, the concave lens m n is interposed; the pencil of rays which would, by the convex glass, converge at x, will now be made to diverge, so as not to come to a focus till they arrive at the retina: and the pencil of rays which would, by the convex glass, have come to a point at y, will, by the interposition of the concave lens, be made to diverge so much as to throw the focus of the rays to b instead of y. By this means, the image of the object is magnified.

Tutor. Can you tell the reason why the tubes require to be drawn out more or less for different persons?

Charles. The tubes are to be adjusted, in or