The total internal reflection of light by a column of water is an experiment that admits of great variety so far as colour is concerned, and is one of the most novel and beautiful experiments with light presented to the public within the last few years. The author had the pleasure of introducing it in the first place at the Polytechnic Institution, where the optical novelty excited the greatest attention, and received the approbation of her Most Gracious Majesty, and his Royal Highness the Prince Consort, with the Royal Family, who were pleased to pay a private evening visit to the Polytechnic, and amongst other things minutely examined the "Illuminated Cascade," which had been erected by Mons. Duboscq of Paris.
The illumination of the descending columns of water was obtained by converging the rays from a powerful electric light upon the orifice from which the water escaped, the Duboscq lantern already explained being employed, and in front of it were placed three cylinders, each having a circular window behind and opposite the lens, and an aperture of about one inch in diameter on the opposite side for the escape of water. The lantern used was of a peculiar shape, and had three sides, the electric light being in the centre of them, and passing through three separate plano-convex lenses to the three cylinders from which the water escaped.
Fig. 282.
Fig. 1. a. The electric light. b c d. The three sides and lenses of the lantern. e f g. The three cylinders of water, each with a circular glass window and orifices at z z z, from which the water and rays of light pass out.—Fig. 2. H. Section of one side of the Duboscq lantern. i i. Cylinder of water, which enters from below. k k. The stream of illuminated water. l l. Bit of coloured glass held between the lantern and the cistern of water.
Attention may be directed to the fact that the light merely passes out of the orifices as a diverging beam of light until the flow of water commences, when the rays are immediately taken up and reflected from point to point inside the arched column of water, and illuminating the latter in the most lovely manner, it appears sometimes like a stream of liquid metal from the iron furnace, or like liquid ruby glass, or of an amethyst or topaz colour, according to the colours of the plates of glass held between the mouths of the lantern and the circular windows in the cylinders of water. The same experiment created quite a furore at the Crystal Palace when it was introduced in one of the author's lectures delivered in that noble place of amusement. In order that our readers may understand the arrangement of the apparatus, we have given at [page 294] a ground plan view of it, as also the appearance of the cascade when exhibited at the Polytechnic to the Royal party. (Fig. 284.)
Fig. 283.
a b. The sides of the cascade. The dotted lines show the reflection of only two rays of the beam of light passing down inside the water.
Another curious effect observed with the illuminated cascade, is the descent of balls of light as the reflection is cut off for a moment by passing the finger through the stream of water, showing that a certain time is occupied in the reflection of light from one end of the cylinder of water to the other; indeed the best idea of the rationale of the experiment is formed by substituting in imagination a silver tube highly polished in the interior, for the descending jet of water. The reflection of sound takes place precisely in the same manner, and the vibrations of the air are reflected from plane, concave, and convex surfaces. It is on this principle that waves of sound thrown off from different surfaces (as of hard rocks), produce the effect of the echo. The sounds arrive at the ear in succession, those reflected nearest the ear being first, and the reflecting surfaces at the greatest distance sending the waves of sound to the ear after the former. At Lurley Falls on the Rhine, there is an echo which repeats seventeen times. Whispering galleries, again, illustrate the reflection of sound from continuous curved surfaces, just as the arched column of water reflects from its interior curved surfaces the rays of light.