a proper distance, between the shutter and the prism. James. How does this apply to the rainbow? Tutor. Suppose A (Plate v. Fig. 32.) to be a drop of rain, and s d a ray from the sun falling upon or entering it at d, it will not go to c, but be refracted to n, where a part will go out, but a part also will be reflected to q, where it will go out of the drop, which acting like a prism, separates the ray into its primitive colours; the violet will be uppermost, the red lowermost. Charles. Is it at any particular angle that these colours are formed? Tutor. Yes, they are all at fixed angles; the least refrangible or red makes an angle with the solar incident ray, equal to little more than 42 degrees; and the violet or most refrangible ray, will make with the solar ray an angle 40 degrees. James. I do not understand which are these angles. Tutor. The ray s d would go to ƒ c, therefore the angle made with the red ray is s fq, and that made with the violet ray is scq; the former 42o 2', the latter 40° 17', Charles. Is this always the case be the sun either high or low in the heavens? Tutor. It is; but the situation of the rainbow will vary accordingly as the sun is high or low, that is, the higher the sun, the lower will be the rainbow: a shower has been seen on a 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 scenes Thomson alludes: it was certainly the most beautiful one I ever beheld: These, when the clouds distil the rosy shower, Charies. 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 T r, to be entering the drop в atr. 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. |